US General Joseph Dunford, the nominee to become chairman of the Joint Chiefs of Staff, has told Congress that Russia poses the greatest security threat to the United States.

Dunford told the US Senate Armed Services Committee on Thursday that Moscow poses a larger threat than the ISIL terrorist group and it is reasonable to send Ukraine heavy weapons to defend against Russian “aggression.”

"My assessment today… is that Russia presents the greatest threat to our national security," said Dunford, who is the current Commander of the Marine Corps.

"So 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 added.

The general also claimed that Russia was a nuclear power capable of violating the sovereignty of US allies in Europe.

However, Dunford acknowledged that despite the challenges in the relationship with Russia, it was important to maintain a good military to military relationship with Moscow “to mitigate the risk of miscalculation.”

US President Barack Obama has nominated Dunford to be the 19th Chairman of the Joint Chiefs of Staff. If confirmed by the Senate as chairman, Dunford will become the highest ranking military officer in the United States Armed Forces.

Dunford's comments echo those made during the past year by General Martin Dempsey, the outgoing chairman of the Joint Chiefs of Staff, who has cited Russia as a top threat.

On Wednesday, US Air Force Secretary Deborah James expressed similar concerns, saying that Russia is “the biggest threat to US Security.”

Relations between the West and Russia have plunged to a post-Cold War low since Moscow reunified with Crimea last year.

The US has accused Russia of destabilizing Ukraine by supporting pro-Russian forces fighting in the eastern parts of the country.

US experts on Russia have said the growing crisis in Ukraine, which has already taken thousands of lives, could explode to a much wider conflict.