Britain and the United States are "concerned" about recent Russian activity on its border with the Ukraine.
The concerns arose as Ukraine's military said two servicemen were killed by pro-Russian separatists in eastern Ukraine.
US State Department spokesperson Ned Price told a press briefing on Monday the concerns came after "credible reports" of Russian troop movements along the borders of Ukraine and occupied Crimea.
He added President Joe Biden had spoken to Ukrainian President Volodymyr Zelensky and the US remained clear in its position to stand by Kiev.
"We have asked Russia for an explanation of these provocations, but most importantly what we have signalled directly with our Ukrainian partners is a message of reassurance," Price told journalists.
British Prime Minister Boris Johnson also spoke with Zelensky on Monday.
"They discussed the UK's significant concerns about the recent Russian military activity on Ukraine's border and in illegally-annexed Crimea," a spokesperson for Johnson said.
"The prime minister reaffirmed our unwavering support for Ukraine's sovereignty and territorial integrity. He expressed his solidarity with President Zelensky's government in the face of this destabilising activity and commended Ukraine's approach to the situation," the spokesperson said.
"They agreed to continue working closely with international partners to monitor the situation and ensure there is no further escalation."
A ceasefire between the Ukrainian army and Russian-backed separatists in Donetsk and Luhansk was agreed at the end of July.
However, since the beginning of the year, at least 14 Ukrainian soldiers have been killed and more than 40 injured. On the separatist side, at least 16 fighters have died, according to a report by the Organisation for Security and Cooperation in Europe.