Russia now poses the greatest threat to US national security and its behavior is "nothing short of alarming," Marine General Joseph Dunford, the nominee to be the US military's top officer, said Thursday.

Dunford, currently the commandant of the US Marine Corps, told senators at his confirmation hearing it would be "reasonable" to provide lethal weapons to Ukrainian forces battling pro-Russian rebels.

But he also said that the Pentagon nevertheless needed to maintain some kind of "effective" military-to-military relationship with Moscow.

"Russia presents the greatest threat to our national security," Dunford, who is expected to replace General Martin Dempsey as chairman of the US Joint Chiefs of Staff, told the Senate Armed Services Committee.

He stressed that Russia is a nuclear power that is capable of violating the sovereignty of US allies and doing "things inconsistent with national interests."

"If you want to talk about a nation that could pose a threat to the United States, I would have to point to Russia. If you look at their behavior, it's nothing short of alarming," Dunford said.

Dunford also cited China, North Korea and the Islamic State group as threats to US national security.

Ties between former Cold War foes Washington and Moscow have plunged to new lows since Russia annexed Ukraine's Crimea region last year. Ukrainian forces are still locked in battle with pro-Russian rebels in the separatist east.

"From a military perspective, I think it's reasonable we provide that support to the Ukrainians," Dunford said.

"Frankly, without that kind of support, they're not going to be able to protect themselves against Russian aggression."

Republican Senator John McCain, who chairs the panel, welcomed Dunford's push for arms.

"In Europe, Vladimir Putin's Russia continues its onslaught in Ukraine," said McCain.

"But even as Russian troops and equipment execute this neo-imperial campaign to undermine Ukraine's government and independence, the United States has refused Ukraine the weapons it needs and deserves for its defense."

The chairman of the Joint Chiefs of Staff acts as the top military advisor to the US president and the secretary of defense.