Washington has said Kiev should not use US arms inside Russia following accusations that pro-Ukraine militia used American armored vehicles.
Chairman of the US Joint Chiefs of Staff Mark Milley said on Thursday his office was looking into images purportedly showing the vehicles used in an attack in the Russian border region of Belgorod.
"I can't say with definitive accuracy right this minute to you whether that - and I saw the same video - whether that's US-supplied equipment or not, what was the nature of the attack, who did what to whom," Milley told a press conference at the Pentagon.
"I can't say that with definitiveness right this minute, but I can say that we have asked the Ukrainians not to use US-supplied equipment for direct attacks into Russia,” he added.
Milley claimed restrictions on US support to Ukraine following Russia's February 2022 military operation was aimed at ensuring Kiev’s self-defense against Russian forces inside Ukraine.
"Why is that? Because we don't want - this is a Ukrainian war. It is not a war between the United States and Russia. It's not a war between NATO and Russia," Milley said.
Moscow has already condemned the use of US-made military hardware in the region.
"It is no secret for us that more and more equipment is being delivered to Ukraine's armed forces," Kremlin spokesman Dmitry Peskov told reporters on Wednesday when asked about the use of US-made hardware.
Milley also said Kiev is unlikely to force out all Russian troops anytime soon.
He said that there are hundreds of thousands of Russian soldiers in Ukraine, which will make it difficult for Kiev to recapture all of its territories.
"That means fighting is going to continue, it's going to be bloody, it's going to be hard. And at some point, both sides will either negotiate a settlement or they'll come to a military conclusion."
Washington has been the prime supporter of Kiev.
US Defense Secretary Lloyd Austin said that the United States and its allies have so far provided a total of $65 billion to Ukraine in security assistance.
The US and its Western allies have been providing Kiev with military equipment worth tens of billions of dollars since Russia began its military campaign in Ukraine in February last year.
Russia has repeatedly warned against flooding Ukraine with weapons, insisting the massive shipments of armaments to Kiev will only prolong the conflict.
Western weapons supplied to Kiev include advanced missile systems, armored vehicles, tanks, and communication systems.
Long-range attack drones have been also added to the list of Western weapons supplied to the Ukrainian forces fighting against Russian troops.