A senior ally of Russian President Vladimir Putin says Russia is not bluffing about using tactical nuclear weapons against Ukraine and warned that the conflict could spill over into other countries.

Dmitry Medvedev, the deputy chair of the Security Council of Russia, made the comments after President Biden quietly authorized Kyiv to launch U.S.-supplied weapons at military targets just over the border in Russia that are supporting an offensive against the northeastern Ukrainian city of Kharkiv.

“This is, alas, neither intimidation nor bluffing,” Medvedev said Friday, speaking on the potential to use strategic nuclear weapons, per Reuters.

UKRAINE SEEKS TO STRIKE RUSSIAN TARGETS WITH WESTERN WEAPONS, ZELENSKYY SAYS

Russia has been using staging locations just across the border to enable its attacks against Ukraine and Biden has given Ukraine the go-ahead to use American weaponry to hit back at Russian forces hitting them or preparing to hit them. Germany has also backed the move.

The White House says the policy is limited and prohibits the use of army tactical missile systems (ATACMS) or long-range strikes inside Russia. 

In March, the U.S. quietly delivered long-range ATACMS to Ukraine for the first time – which the Ukrainians have since deployed against Russian military forces inside Ukraine.

Medvedev said Friday that “Russia regards all long-range weapons used by Ukraine as already being directly controlled by servicemen from NATO countries.”

“This is no military assistance, this is participation in a war against us. And such actions could well become a casus belli (an act that provokes a war),” Medvedev said Friday, per Reuters. 

Medvedev, who served as Russian president from 2008 and 2012, said that the West’s ongoing support of Ukraine could lead to an escalation of the 27-month-old full-scale invasion.

“The current military conflict with the West is developing according to the worst possible scenario. There is a constant escalation when it comes to the firepower of NATO weapons being used. Therefore, nobody today can rule out the conflict’s transition to its final stage,” Medvedev said.

KYIV’S FORCES ARE UP AGAINST A CONCERTED RUSSIAN PUSH IN EASTERN UKRAINE, A MILITARY OFFICIAL SAYS

The comments come as depleted Ukrainian troops are losing ground in the war – and just weeks after the U.S. agreed to send an extra $60 billion in aid to the war-torn country. In the border region of Kharkiv, Ukraine has endured a Russian onslaught this month that has stretched Kyiv’s outgunned and outmanned forces.

The White House says that Russia’s forward progress has stalled and that Russia will not be able to capture Kharkiv. 

Russia has only moved forward by a few kilometers and its forces are under relentless barrage by the Ukrainians and suffering at an extraordinary cost, the White House tells Fox News. 

Ukrainian President Volodymyr Zelenskyy said Friday that it’s only a matter of time before Ukraine utilizes the Western weaponry to strike Russian territory.

The developments and threats of escalation came just weeks after Gen. Charles Brown, the Chairman of the Joint Chiefs of Staff, said NATO military trainers will eventually be sent to Ukraine, according to a report in the New York Times.

Ukrainian officials have asked their U.S. and NATO counterparts to help train 150,000 new recruits closer to the front line for faster deployment, per the report. 

Rep. Eli Crane, R-Ariz., told Fox News Digital at the time that deploying military trainers would lead to a wider war in the region.

Friday’s comments by Medvedev are not the first time he has taken a hardline stance against the West. In January, he warned the U.K. that putting boots on the ground in Ukraine would amount to a declaration of war against Russia.

In January, he also raised the prospect of nuclear war, warning NATO allies that a defeat for Russia in Ukraine could provoke a nuclear war. 

“The loss of a nuclear power in a conventional war can provoke the beginning of a nuclear war,” he said in a Telegram post.

“Nuclear powers have [never] lost major conflicts on which their fate depends,” the Kremlin official added. 

Fox News’ Jennifer Griffin, as well as Reuters and The Associated Press contributed to this report.