President Barack Obama's nominee for defense secretary said Wednesday he would likely support delivering arms to Ukraine to counter pro-Russian separatists, but the White House made clear the final decision would rest with the commander-in-chief.

Ashton Carter, expected to be confirmed soon by lawmakers as the next Pentagon chief, offered his view to the Senate Armed Services Committee amid signals the Obama administration is leaning towards providing weapons to Kiev.

"I'm very much inclined in that direction, Mister Chairman, because I think we need to support the Ukrainians in defending themselves," Carter said when asked by Republican Senator John McCain if he supported delivering "defensive weapons" to Kiev.

"The nature of those arms, I can't say right now," Carter noted, explaining he had not conferred with US military commanders or Ukrainian leaders on the issue.

The Obama administration so far has only provided "non-lethal" assistance to Ukraine, including night-vision goggles, body armor, radios and other items, while imposing economic sanctions on Russia over its role in the conflict.

When asked about Carter's comments, the White House underlined that the president -- and not Carter -- would make the final call on the issue.

"The other thing that I noticed, that Doctor Carter mentioned in his hearing, is that he is a strong believer in the chain of command and he certainly understands that the commander-in-chief is the top of that chain of command and that a decision like this will be made by the commander-in-chief," White House spokesman Josh Earnest told reporters.

"That said, the president is certainly interested in the view, the opinion and the insight of his national security team, including what we hope will be his soon-to-be-confirmed new secretary of defense, and the president will certainly take that advice into account," he said.

- Deter Russian 'adventurism' -

The Obama administration has feared that sending in weapons could trigger a tense confrontation with Russia and escalate the conflict.

But with no sign that Moscow is backing off in Ukraine and repeated violations of a ceasefire agreement, US officials say they are now seriously considering a shift in policy.

Some top US commanders and senior figures in the administration favor the move, following repeated appeals from Kiev, officials said.

Washington has accused Russia of running a proxy war in Ukraine, providing advice and heavy weapons to the pro-Moscow separatists.

A group of former senior US officials and retired officers this week urged a major increase in military assistance for Ukraine, including providing light-armor missiles designed to take out tanks as well as radar to counter artillery fire.

If confirmed by the Senate as the next defense secretary, Carter said an "early" priority would be to ensure the United States was "doing enough" to reassure NATO allies in the Baltics and across eastern Europe.

"I understand that we are adding forces, rotational forces to the Baltic states as a presence there, as a deterrent to any Russian kind of adventurism," he said.

"I certainly support that, and if I'm confirmed, I'd want to look into what more we can do to do that."

Carter also recalled witnessing the signing of a landmark 1994 agreement, the Budapest Memorandum on Security Assurances, in which Ukraine relinquished its nuclear arsenal.

The deal, which was signed by Moscow, "provided for Russia to respect the territorial integrity of Ukraine, which ... it has obviously not done," he said.

"And that was a part of the climate and context in which the Ukrainians agreed to give up nuclear weapons in the first place."

US troops are due to deploy to Ukraine this spring to train units from Kiev's national guard. Washington has promised $120 million in non-lethal military aid, with about half already delivered.

Carter, a former academic who has served in top jobs at the Pentagon, was nominated in December to replace Defense Secretary Chuck Hagel, who resigned under pressure from the White House.