General Mark Milley, who was nominated to be the next Army chief of staff, said he agreed with Marine Corps Commandant Joe Dunford that Russia poses the greatest threat to the United States because of its large nuclear arsenal and its "very, very aggressive" behavior since 2008.
"They've attacked and invaded Georgia, they've seized the Crimea, they've attacked into the Ukraine," he told the Senate Armed Services Committee. "That's worrisome."
In view of the bombardment of Ukraine by artillery and rocket fire from Russian-backed rebels since last year, Milley said, "I would be in favor of [providing Kyiv with] lethal defensive equipment."
The United States has resisted equipping Ukraine with lethal weapons previously due to concerns about escalating the conflict. But Milley's statement is the latest sign that the Pentagon may now be considering it.
Milley said he also favors temporarily increasing and rotating U.S. ground forces in Europe to reassure allies and deter Russian aggression. The Army is already prepositioning equipment and engaging in exercises.
Milley, who served in both Iraq and Afghanistan, said U.S. air controllers may need to be stationed with Iraqi forces close to combat areas to increase the effectiveness of air strikes on Islamic State targets.
That would ensure "more effective close-air support" for Iraqi troops, which have been struggling to regain and hold ground taken by the militant Islamist group in northwestern Iraq since last year.
Top brass has previously rejected routine use of forward air controllers to direct the hundreds of air strikes by U.S. and allied forces against IS every month, out of concern about the possibility of combat casualties.
Despite that, Milley said the United States also should consider having military advisers in Iraq "go forward with units" into combat areas to help Iraqi forces, though "there are lots of issues with the security of our people and the risks associated with that."
Separate, Vice President Joe Biden and Iraqi Prime Minister Haidar al-Abadi spoke about the recent delivery of F-16 fighter jets, calling it a "major milestone" in the partnership between the two countries, the White House said.
Biden offered condolences for the attack last week that killed more than 100 people in a suicide car bombing in Diyala province, and the leaders discussed the ongoing campaign against Islamic State militants in Anbar province, the White House said.