US Secretary of State Antony Blinken has spoken to French Foreign Minister Jean-Yves Le Drian about "concerning" Russian military activity "in and near Ukraine."
The two diplomats discussed "reports of concerning Russian military activity in and near Ukraine and their continued ironclad commitment to Ukraine's sovereignty and territorial integrity", US State Department spokesman Ned Price said.
Western allies have voiced mounting alarm over recent Russian troop movements near the country's border with Ukraine.
The Ukrainian army has been locked in a simmering conflict with Kremlin-backed separatists in the Donetsk and Lugansk regions that erupted after Russia annexed the Crimean peninsula in 2014.
Kiev said two Ukrainian soldiers were killed on Friday by an explosive device in the country's separatist east, while recent social media videos show Russia moving troops, tanks and missiles towards the Ukrainian border.
After an uptick in violence at the beginning of the year, Russia amassed around 100,000 troops on Ukraine's borders in the spring, raising fears of a major escalation in the conflict.
Russia later announced it was pulling back but both Ukraine and its ally the US said at the time that the withdrawal was limited.
The conflict has claimed more than 13,000 lives.
Ukraine and its Western allies accuse Russia of sending troops and arms across the border. Moscow has denied those claims.