On two separate occasions last year, railroad cars carrying tens of thousands of kilograms of smokeless powder — enough propellant to collectively make at least 80 million rounds of ammunition — rumbled across the China-Russia border at the remote town of Zabaykalsk.

The powder had been shipped by Poly Technologies, a state-owned Chinese company on which the United States had previously imposed sanctions for its global sales of missile technology and providing support to Iran. Its destination was Barnaul Cartridge Plant, an ammunition factory in central Russia with a history of supplying the Russian government.

These previously unreported shipments, which were identified by Import Genius, a U.S.-based trade data aggregator, raise new questions about the role China has played in supporting Russia as it fights to capture Ukrainian territory. U.S. officials have expressed concerns that China could funnel products to Russia that would help in its war effort — what is known as “lethal aid” — though they have not said outright that China has made such shipments.

Speaking from Beijing on Monday, Antony J. Blinken, the U.S. secretary of state, said China had assured the United States that it was not providing lethal assistance to Russia for use in Ukraine, and that the U.S. government had “not seen anything right now to contradict that.”

“But what we are concerned about is private companies in China that may be providing assistance,” Mr. Blinken said.

Some experts said the shipments Poly Technologies had made to Barnaul Cartridge Plant since the invasion, which totaled nearly $2 million, according to customs records, constituted such lethal assistance. According to the customs records, Poly Technology intended its shipments to be used in the kinds of ammunition fired by Russian Kalashnikov assault rifles and sniper rifles.

William George, the director of research at Import Genius, said that Poly Technologies “may be toeing the line on exactly what constitutes lethal aid to Russia,” but that the implications of the shipments were clear.

“When shipping large quantities of gunpowder intended for the creation of military cartridges to a country at war, it’s unreasonable to imagine that the finished product won’t be used to lethal effect on the battlefield,” Mr. George said.

“It is lethal support,” said Alexander Gabuev, director of the Carnegie Russia Eurasia Center. “The question is, how impactful and large scale is that?”

Mr. Gabuev said that China had generally refrained from any actions that would “in a visible, forceful way” cross red lines the U.S. government had detailed at the beginning of the war about what would constitute a violation of Western sanctions. Since Poly Technologies has a history of shipments to the Barnaul plant before the war though, China might see those shipments as part of regular trade flows.

“By and large, China tries to stick to those red lines,” he said. “Having said that, we see that there are some contracts and transactions going on.”

Poly Technologies is a subsidiary of China Poly Group Corporation, which is owned by the Chinese government. Previous reports by The Wall Street Journal and CNN documented shipments of navigation equipment and helicopter parts from Poly Technologies to Russian state-backed firms.

Barnaul Cartridge Plant, the recipient of the powder shipments, is privately owned. But Russian procurement records provided to The New York Times by C4ADS, a Washington, D.C.-based global security nonprofit, show the company had numerous contracts with divisions of the Russian government and military over the past decade, including the Russian Ministry of Defense.

Barnaul Cartridge Plant was added to a list of companies sanctioned by the European Union in December. Open source information suggests the plant may have served as a training camp linked with the Wagner Group, a private Russian military force with ties to Russian President Vladimir V. Putin.

There is no known direct link between these particular shipments of smokeless powder and the Ukrainian battlefield, and in customs paperwork Poly Technologies described the powder as being “for assembly of foreign-style hunting cartridges.”

But Brian Carlson, a China-Russia expert and the head of the global security team of the think tank at the Center for Security Studies, said that while such cartridges could be used for hunting, this was rare. “These are military cartridges,” he said.

Most modern firearms and other weapons used by soldiers and civilians alike rely on smokeless powder to propel a bullet to its target. When the trigger is pulled, a firing pin strikes the rear of the ammunition cartridge, igniting the powder, which burns extremely fast and forces the bullet down the barrel of a firearm.

This kind of powder is also used by militaries as the propellant for mortar ammunition, launching explosive-laden projectiles weighing from four pounds to 30 pounds or more.

Poly Technologies and Barnaul Cartridge Plant did not respond to requests for comment.

The war in Ukraine, now in its 17th month, has intensified in recent weeks. The ability of both militaries to obtain munitions and equipment has become a crucial factor that could influence the war’s outcome.

Western countries clamped down on their trade with Russia following the invasion, to try to starve the country of military goods as well as supplies that feed their economy and help the government generate revenue.

But countries like China, India, the United Arab Emirates, Kyrgyzstan and Turkey stepped in to provide Russia with goods ranging from mundane products like smartphones and cars to aircraft parts and ammunition.

Both state-owned and private Chinese companies have sold Russia products that could plausibly be used by either civilians or the military — including drones, semiconductors, hunting rifles, navigation equipment and airplane parts.

China has remained officially unaligned in the war. Officials there argue Beijing is a neutral party and a peacemaker. In practice, however, China has become an important diplomatic, economic and security partner for Russia, after proclaiming a “no limits” partnership early last year.

In a speech in April in Washington, Treasury Secretary Janet L. Yellen called that partnership a “worrisome indication” that China is not serious about ending the war. And she warned that the consequences for China of providing Russia with material support or assisting in evading sanctions “would be severe.”

In recent months, U.S. officials have also privately reached out directly to Chinese financial institutions to discuss the risks of facilitating the evasion or circumvention of sanctions and export controls.

Chinese companies “have a choice to make,” Wally Adeyemo, the deputy Treasury secretary, said in an interview on Fox Business TV earlier this month. “They can provide Russia with material support for their military and continue to do business with an economy that represents maybe $1.5 trillion and is getting smaller, or you can continue to do business with the rest of the world.”

Poly Technologies is one of China’s largest arms exporters. It produces equipment for police and military forces, including weapons, personal protective gear, explosives and missile systems. It attracted censure in past decades for shipping small arms to Zimbabwe. In the last few years, it has sent weapons shipments to Pakistan, Sri Lanka and Nigeria, according to records accessed through Sayari Graph, a mapping tool for corporate ownership and commercial relationships.

Barnaul products have been common on American shelves in recent years, including ammunition for military-style rifles, hunting rifles and American handguns. The goods came to America through several importers, including MKS Supply, LLC, a wholesale ammunition distributor in Dayton, Ohio.

According to an MKS Supply official, the company stopped working with Barnaul Cartridge Plant early last year following a U.S. government ban on imports of Russian ammunition.

Edward Wong contributed reporting from Beijing.