During his confirmation hearing to become chairman of the Joint Chiefs of Staff, Marine Gen Joseph Dunford called Russia's recent actions "nothing short of alarming".
Relations between the US and Russia have deteriorated since Russia annexed Ukraine's Crimea region last year.
A pro-Russian insurgency in eastern Ukraine has also prompted US sanctions.
"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." Gen Dunford told senators on Thursday.
Some US lawmakers have pushed for the military to provide lethal weapons to Ukraine's government, a move Gen Dunford called "reasonable".
"Frankly, without that kind of support, they are not going to be able to defend themselves against Russian aggression," Gen Dunford said.
Thus far the US has only provided non-lethal equipment and training for Ukrainian forces.
More than 6,000 people have been killed in fighting in Ukraine, which began in April 2014, between government forces and pro-Russian rebels in the eastern Donetsk and Luhansk regions.
Russia denies arming the rebels or sending troops there.
A general in line for the highest US military post says Russia poses the greatest threat to national security.