NATO's top commander told House lawmakers Wednesday that even with U.S.-supplied lethal weaponry, Ukraine's military forces may not be able to stop Russia's advance in the eastern part of that country.

Air Force Gen. Philip Breedlove, NATO supreme allied commander in Europe and chief of U.S. European Command, told members of the House Armed Services Committee that he has discussed the possibility of supplying lethal weapons to Ukraine with White House officials, including what kinds of arms Ukraine has requested.

Small arms could be provided in a "very short timeline," Breedlove said, but heavier weaponry would need to go hand-in-hand with advanced training.

Even so, Breedlove said, "I do not believe Ukrainian forces can stop a Russian advance in eastern Ukraine."

Lawmakers at the hearing acknowledged that Russia's "invasion of Ukraine" is worrisome, but also expressed concerns about Moscow's efforts to expand its nuclear capabilities and refine its nuclear doctrine.

Christine Wormuth, the Pentagon's undersecretary for policy, who accompanied Breedlove at the hearing, noted that Russia "is in the process of evolving this doctrine in a matter of ways ... that are concerning to us."

Rep. Mike Turner, R-Ohio, criticized the Defense Department's efforts to downsize its nuclear arsenal while it "remains clear" that Russia has been in violation of accords to reduce its own nuclear capability.

Turner said Russia has most likely "had the ability to increase, while we had to cut."

Wormuth and Breedlove both said that while it remains to be seen what Russia's precise agenda is in that regard, a more immediate concern is how to deter Moscow from encroaching on other NATO territories that hold substantial Russian populations.

Breedlove said he supports a more robust permanent troop presence in U.S. European Command, not only to act as a counterweight to Russia but also to help deter rising threats such as the Islamic State group and missions in the U.S. Africa Command theater of operations.

Breedlove reiterated that additional U.S. forces will rotate throughout Europe over the next year, but he did not say if those forces would be involved in providing direct military assistance to Ukraine in some way.