US, Ukraine urge end to soaring tensions with Russia

US Vice President Joe Biden and Ukraine's President Petro Poroshenko in a phone call urged a speedy resolution to the conflict with pro-Russian separatists amid a spike in tensions with Moscow, the White House has said.

Biden and Poroshenko "expressed concern over the recent surge in fighting in eastern Ukraine, where ceasefire violations by combined Russian-separatist forces are at their highest levels since 2015, often using heavy weapons", the White House statement said on Friday.

Kiev and the West accuse Russia of supporting the separatist rebels in eastern Ukraine and deploying troops across the border, claims Moscow denies.

Biden and Poroshenko "reiterated the need for a political and diplomatic resolution of the conflict through full implementation of the Minsk agreements by all parties," the statement added.

The Minsk accord, signed in February 2015 with French and German mediation and in the presence of President Vladimir Putin, calls for a ceasefire along with a range of political, economic and social measures to end the conflict.

Biden told Poroshenko that US officials "had sent a message to Russia that the world is watching and underscored the need to deescalate the situation."

He also urged Ukraine to show restraint.

Poroshenko on Thursday said he could not rule out a "full-scale" Russian invasion as violence raged in the east, amid accusations that Russia is building up its military force in the region.

The Pentagon on Friday said that the extra Russian troops along the border were associated with a regular military exercise.

Poroshenko's claim came after Russian President Vladimir Putin lashed out last week at Kiev over an incident on the frontier between Crimea, which Moscow annexed in March 2014, and Ukraine, accusing it of "practising terror".

Two Russian officers were killed in the incident, in which Ukraine denied any involvement.

Putin on Friday visited Crimea to boost security measures there. It was his fifth visit to the Black Sea peninsula since annexing it from Ukraine.

Separately, Biden and Poroshenko discussed conditions set by the International Monetary Fund for disbursing the next tranche of much-needed financial assistance, the White House said.

They also "agreed on the importance of recent Ukrainian efforts to continue critical anti-corruption reforms", the statement said.