WASHINGTON (AFP) - The United States on Thursday (April 1) warned Russia against "intimidating" Ukraine, which said that Moscow has been building up troops on its border.

"We're absolutely concerned by recent escalations of Russian aggressive and provocative actions in eastern Ukraine," State Department spokesman Ned Price told reporters.

"What we would object to are aggressive actions that have an intent of intimidating, of threatening, our partner Ukraine."

Ukrainian and US officials have reported troop movements in Crimea, which Russia annexed from Ukraine, as well as areas of the border Donbass region under control of Moscow-backed separatists.

Both US Defence Secretary Lloyd Austin and Secretary of State Antony Blinken have called their Ukrainian counterparts to stress support.

Blinken in a call to Foreign Minister Dmytro Kuleba "affirmed the United States' unwavering support for Ukraine's sovereignty and territorial integrity in the face of Russia's ongoing aggression in the Donbass and Crimea," Price said.