By Phil Stewart and David Alexander
WASHINGTON (Reuters) - Russia presents the greatest threat to U.S. national security and its behavior is "nothing short of alarming," Marine General Joseph Dunford told lawmakers on Thursday as they weighed his nomination to become chairman of the Joint Chiefs of Staff.
Dunford also added his voice to those Pentagon officials who have supported providing lethal arms to Ukraine to help it defend itself from Russia-backed separatists, a step that President Barack Obama has so far resisted.
"My assessment today, Senator, is that Russia presents the greatest threat to our national security," said Dunford, the Marine Corps commandant, who is expected to swiftly win Senate confirmation to become the top U.S. military officer.
Relations between Moscow and the West have plunged to a post-Cold War low since Russia's intervention in Ukraine upended assumptions about the security of NATO's eastern flank.
"If you want to talk about a nation that could pose an existential threat to the United States, I'd have to point to Russia. And if you look at their behavior, it's nothing short of alarming," he said.
In Dunford's view, Russia is at the top a list of security concerns that also included China, whose rapidly expanding military has alarmed Pentagon officials, North Korea and the threat from Islamic State militants.
Without weapons to counter tank and artillery fire, Ukrainian troops would not be able to fend off "Russian aggression," he said.
"From a military perspective it's reasonable that we provide that support to the Ukrainians," Dunford said.
U.S. Defense Secretary Ash Carter has also said he was inclined to provide defensive arms to Ukraine. But Carter has qualified that position, saying economic sanctions should be the primary focus for exerting pressure on Moscow.
The wide-ranging hearing covered everything from the Marine Corps general's views on the conflict in Syria to the growing U.S. military focus on the Asia-Pacific and to how he got his nickname, "Fighting Joe."
Asked whether Obama's strategy of pursuing a multisectarian Iraq would be successful, Dunford acknowledged the country is beset by a deep sectarian divide that is fueling the Islamic State movement.
A unified, multisectarian government Iraq "is going to be very difficult," he said, adding however, that it still represented the best prospect for long-term success.
Republicans have slammed Obama for announcing plans to slash U.S. troops in Afghanistan to an embassy presence in 2017. Dunford, the former commander of U.S. forces in Afghanistan, acknowledged that timelines don't always hold.
"That's certainly the case in Afghanistan," he said.
(Reporting by Phil Stewart and David Alexander; Editing by Doina Chiacu and Christian Plumb)