Ukrainian President Volodymyr Zelenskiy on Thursday accused Moscow of building up troops on his country's border as the United States warned Russia against "intimidating" Ukraine.
Kiev has been locked in a conflict with Russian-backed separatists since 2014, and this week Ukrainian officials reported Russian troop movement in annexed Crimea and on the border, near territories controlled by Moscow-backed separatists.
On Thursday, Zelenskiy's ministers discussed the escalating security situation with Western allies including U.S. Secretary of Defense Lloyd Austin.
"Muscle-flexing in the form of military exercises and possible provocations along the border are traditional Russian games," Zelenskiy said in a statement.
He accused Moscow of seeking to create "a threatening atmosphere" as Kiev hopes to resume a ceasefire brokered last year.
The U.S. State Department said it was "absolutely concerned by recent escalations of Russian aggressive and provocative actions in eastern Ukraine."
"What we would object to are aggressive actions that have an intent of intimidating, of threatening, our partner Ukraine," State Department spokesman Ned Price told reporters.
Some observers say the reported Russian troop buildup is a test for the administration of U.S. President Joe Biden, who caused an uproar in Moscow last month by calling his Russian counterpart Vladimir Putin a "killer."
This week, Moscow and Kiev blamed each other for a rise in violence between government forces and Kremlin-backed separatists in eastern Ukraine, which has undermined the ceasefire.
Zelenskiy said 20 Ukrainian servicemen had been killed and 57 wounded since the start of the year.
Separately, the military announced that a Ukrainian soldier was wounded in an attack it blamed on separatists.
On Thursday, U.S. Secretary of Defense Austin called his Ukrainian counterpart Andriy Taran, Ukraine's defence ministry said.
Austin said during the call that Washington would "not leave Ukraine alone in the event of escalating Russian aggression," the ministry said.
Ukrainian Foreign Minister Dmytro Kuleba for his part discussed the "aggravation by the Russian Federation of the security situation" on the frontline with his Canadian counterpart Marc Garneau.
Ukraine's military intelligence accused Russia of preparing to "expand its military presence" in the separatist-controlled eastern regions of Donetsk and Lugansk.
In a statement, the intelligence service said it "does not rule out" an attempt by Russian forces to move "deep into Ukrainian territory."
A high-ranking Ukrainian government official, speaking on condition of anonymity, claimed that the Russian army was practising "military coordination" with separatists.
"From mid-April their combat units will be ready for an offensive," the official told AFP.
West should not 'worry'
Moscow has repeatedly denied sending troops and arms to buttress the separatists and Putin's spokesman stressed on Thursday that Moscow is at liberty to move troops across its territory.
"The Russian Federation moves its armed forces within its territory at its discretion," spokesman Dmitry Peskov told reporters, but he did not directly confirm a troop buildup on the Ukrainian border.
He added that "it should not worry anyone and does not pose a threat to anyone."
The war in eastern Ukraine broke out in 2014 when Russia annexed the Crimean peninsula following a bloody uprising that ousted Ukraine's Kremlin-friendly president Viktor Yanukovych.
On Wednesday, the Pentagon said U.S. forces in Europe had raised their alert status following the "recent escalations of Russian aggression in eastern Ukraine."
Mark Milley, chairman the U.S. Joint Chiefs of Staff, also spoke with his Russian and Ukrainian counterparts, Valery Gerasimov and Ruslan Khomchak.
Khomchak said this week that 28,000 separatist fighters and "more than 2,000 Russian military instructors and advisers" are currently stationed in eastern Ukraine.
On Thursday, the deputy head of Zelenskiy's office, Roman Mashovets, called for joint drills with NATO forces to "help stabilize the security situation."
Zelenskiy was elected in 2019 promising to end the years-long conflict, but critics say a shaky ceasefire was his only tangible achievement.
The fighting has claimed more than 13,000 lives since 2014, according to the United Nations.