A commander of an Iranian-backed militia has been charged with plotting to attack Jewish sites in the United States, including a synagogue in New York City, and carrying out terror attacks in Europe as part of a broader campaign of retaliation by Iran since the war began in February.

A criminal complaint unsealed on Friday accused the commander, Mohammad Baqer Saad Dawood al-Saadi, of planning at least 20 attacks against U.S. and Israeli interests in Europe and Canada since late February. Mr. al-Saadi was detained in Turkey recently and handed over to U.S. authorities, Mr. al-Saadi’s lawyer said in federal court in Manhattan on Friday. Mr. al-Saadi appeared in court on Friday, but did not enter a plea.

Mr. al-Saadi, according to the complaint, is a commander of Kataib Hezbollah, an Iraqi militia that is a proxy for the Islamic Revolutionary Guards Corps and has helped Tehran project power across the region, including through attacks on American forces and diplomatic targets. While Kataib Hezbollah has long been one of the most important groups serving as an armed proxy for Iran in the Middle East, it does not have a history of organizing attacks outside the region.

Since the U.S. and Israeli militaries attacked Iran in February, the authorities in Europe and the United States have heightened security, especially at Jewish sites, warning of retaliation. The Iranian government’s forces have carried out attacks on U.S. military sites in the Middle East and targeted infrastructure in Arab nations closely aligned to the United States.

The complaint directly tied Mr. al-Saadi, 32, to an obscure group called Harakat Ashab al-Yamin al-Islamiyya that emerged in March. The group took credit for attacks in London, Belgium and the Netherlands without disclosing its links to the Iraqi militia.

At the time, counterterrorism officials said they were investigating whether the group had ties to Iran and whether the attacks were part of a wave of low-cost, unsophisticated methods to sow fear in Jewish communities across Europe. The people accused of carrying out the European attacks do not appear to be linked to extremist groups and were most likely recruited with promises of money, according to authorities and lawyers.

But U.S. authorities, according to the complaint, say Harakat Ashab al-Yamin al-Islamiyya was a front for Kataib Hezbollah, a U.S. designated terrorist group, and investigators noted the similarities between their logos.

“Essentially overnight, Harakat Ashab al-Yamin al-Islamiya was able to activate terrorist cells across Europe” in response to the war in Iran, according to the complaint, which was signed by a special agent of the Federal Bureau of Investigation. The violence stoked a new wave of anxiety for European Jews who were fearful that their communities would be targeted.

The arrest came as President Trump weighed renewing strikes to force Iran to meet his demands. Since late February, the strikes have focused on killing Iran’s top leaders, including the ayatollah, and disrupting the government’s abilities to operate terrorist networks across the region.

Since Mr. Trump’s first term in office, the Department of Justice has said it has disrupted numerous plots linked to Iran, including at least one aimed at killing the president, but not directly organized or carried out by Iran or its proxies.

From its inception, the Kataib Hezbollah has been closely tied to Iran’s Quds Force — the overseas arm of the powerful Revolutionary Guards. It made evicting U.S. forces from Iraq a primary focus. Kataib Hezbollah’s repeated attacks on U.S. Army posts in Iraq and Syria over the years contributed to Washington’s decision in 2009 to designate it as a foreign terrorist organization.

The complaint says that Mr. al-Saadi planned to kill “Americans and Jews” in Los Angeles and Arizona and that he had started planning an attack on a synagogue in New York City.

Mr. al-Saadi is one of the highest level figures tied to Iran known to have been arrested by the United States since the start of the war. For years, and during the current conflict, the United States and Israel have focused on killing Iranian officials.

As a leader of Kataib Hezbollah, Mr. al-Saadi worked with Maj. Gen. Qassim Suleimani, the head of Iran’s security machinery, according to the complaint. The U.S. military killed Mr. Suleimani in a drone strike in 2020.

Mr. al-Saadi’s case appears to involve the kind of retaliatory act of terrorism that U.S. officials have long anticipated and feared.

Jay Clayton, the U.S. attorney in Manhattan, said, “Al-Saadi attempted to disrupt American society through intimidation and violence. In a righteous and just contrast, his prosecution will highlight the best of our country.”

Speaking at a Friday night prayer service at Temple Emanu-El on Manhattan’s Upper East Side, one of the country’s largest Reform synagogues, the New York police commissioner, Jessica S. Tisch, said the plot in New York had intended to target “the heart of our Jewish community.”

She did not identify the synagogue, but said that Mr. al-Saadi had chosen it because of its support for Israel. Officials from her department were working with the leaders of the synagogue to ensure its safety, she said.

Since March 9, the complaint says, there have been attacks against synagogues in Liège, Belgium and Rotterdam, Netherlands, and against a Jewish school in Amsterdam. There have been attacks against the Bank of New York Mellon in Amsterdam and the Bank of America building in Paris, which are both U.S. based companies.

In London, there was an arson attack on four Hatzalah ambulances, which serve primarily Orthodox Jewish communities, and in late April in that city, an individual stabbed and seriously injured two Jewish men — including a dual U.S.-British citizen.

The complaint says Mr. al-Saadi worked closely and in person with Mr. Suleimani, who led the Quds Force before his death in the drone strike. Mr. al-Saadi also worked closely with the Iraqi militant leader who led Kataib Hezbollah, known by the nom de guerre Abu Mahdi al-Muhandis, who was also killed in the same drone strike, the complaint says.

Mr. al-Saadi, in a March 20 phone call, discussed with an individual the European terrorist attacks and said he sought help in planning additional attacks in the United States and that he was willing to kill people in them, according to the complaint. He referred to “Ashab al-Yamin” on the call, but said they did not need help in Europe. “Things are working for us here in Europe,” he said.

The complaint describes the person Mr. al-Saadi spoke with only as “a source of information for the F.B.I.” — or SOI-1, who recorded the call and has voluntarily provided information to the bureau about Mr. al-Saadi.

The complaint says Mr. al-Saadi claimed he was “running multiple teams” and asked whether SOI-1 could help carry out attacks in the United States and in Canada, against targets like “Jewish synagogues” and “Zionist headquarters.”

In an April 1 call, Mr. al-Saadi asked SOI-1 for someone who could carry out such attacks and how much that person would want to be paid, the complaint says. SOI-1 said he was able to find “a Mexican person” who was willing to carry out a bombing operation.

According to the complaint, Mr. al-Saadi said during the call: “This war will not end. Either they eradicate us, or we eradicate them.”

In a call two days later, SOI-1 introduced Mr. al-Saadi to an undercover law enforcement officer who was posing as a Mexican cartel member who SOI-1 said could carry out a terrorist attack in New York and elsewhere in the United States.

That same day, Mr. al-Saadi texted the purported cartel member a photograph and a map showing the location of a prominent Jewish synagogue in Manhattan and an Arabic document describing its congregation as one supporting “the right for Israel to exist,” the complaint says.

Mr. al-Saadi also sent photos and maps of additional potential targets: two prominent Jewish centers in Los Angeles and Scottsdale, Ariz.

The complaint says Mr. al-Saadi agreed to pay $10,000 for the attacks, and on April 4, he paid the purported cartel member $3,000 in cryptocurrency in anticipation of the synagogue attack in New York.

The complaint says that on April 6, Mr. al-Saadi relayed a message to the purported cartel member through SOI-1 that “the operation needs to happen TODAY.”

That evening, the purported cartel member sent Mr. al-Saadi a video showing there was a police presence on the block where the synagogue was located. About an hour later, Mr. al-Saadi asked for an update on the status of the attack, but did not receive a response, the complaint says.

The next morning, it says, Mr. al-Saadi again asked the purported cartel member what had happened, and why the operation had not been completed.

Mr. al-Saadi was charged with, among other counts, conspiring to provide material support to Kataib Hezbollah and the Revolutionary Guards and conspiracy to bomb a place of public use.

On Friday, Mr. al-Saadi entered the courtroom shortly after 1 p.m. wearing army green pants and a black shirt. He nodded as a magistrate judge, Sarah Netburn, read the charges, and he then spoke animatedly with his lawyer through an interpreter. The lawyer, Andrew Dalack, told the judge that his client understood the charges and wanted the court to know that he was a political prisoner.

As the hearing came to a close, Mr. Dalack said his client wanted him to reiterate that it was “very important” for the court know that “he is a political prisoner and prisoner of war and should be treated as such.”

Euan Ward and Adam Goldman contributed reporting.