Russian troops near the Ukraine border will return to base in at least the next 20 days following an order from President Vladimir Putin, the military's chief of general staff said Friday.

"The troops are being moved onto trains and military transport aircraft. This will take around 20 days," General Valery Gerasimov, Russia's chief of general staff, was quoted as saying by RIA Novosti news agency.

The United States on Friday offered a cautious response to Russian President Vladimir Putin's statement that he would respect the result of Ukraine's weekend election.

"We would welcome an indication from Russia that they would accept the results of a free and fair and democratic election in Ukraine," said White House spokesman Jay Carney.

Russian President Vladimir Putin pledged Friday to respect the outcome of Ukraine's presidential election but warned that the country had descended into all-out civil war after a bloody upsurge in separatist violence.

At least seven people were killed in fighting between rebels and defense forces outside the eastern industrial hub of Donetsk on Friday, a day after the deaths of 18 soldiers in the heaviest loss for the Ukraine military since the conflict began.

Russian investigators on Friday launched a probe into three associates of top opposition leader Alexei Navalny for alleged fraud during his Moscow mayoral campaign.

The Russian Investigative Committee said in a statement that the probe targets three Navalny associates -- Moscow city councillor Konstantin Yankauskas, and businessmen Nikolai Lyaskin and Vladimir Ashurkov -- accused of embezzling funds gathered for his campaign.

At least five people were killed Friday in fighting near Ukraine's eastern hub of Donetsk two days before a presidential election undermined by an upsurge in attacks by pro-Russian separatists.

The latest bloodshed on the heels of the deaths of 18 soldiers Thursday in the eastern rustbelt near Russia underscores the trouble the interim leaders have in making sure they can pull off a safe and well-attended vote Sunday.

Russian Foreign Minister Sergei Lavrov said on Friday the West needed to learn the "right lesson" from the Ukraine crisis, as he criticized efforts to contain Russia since the collapse of the USSR.

"If the goal is to prevent such crises from repeating in the future, the right lesson must be learned from events in Ukraine," Lavrov told a security conference in Moscow. 

German Chancellor Angela Merkel on Friday called on Russian President Vladimir Putin to accept the OSCE's assessment on Ukrainian presidential elections this weekend.

"I expect Russia to respect the doubtlessly objective assessment of the OSCE -- after all it is a member of the organisation," Merkel told newspaper the Saarbruecker Zeitung.

China and Russia vetoed Thursday a draft U.N. Security Council resolution to refer Syria to the International Criminal Court for crimes committed by both sides in the three-year civil war.

Western powers pressed for the resolution in the face of mounting atrocities in Syria, including chemical attacks, systematic torture, barrel bombings and blocked aid access.

Pro-Russian rebels firing mortar shells and grenades killed 14 Ukrainian soldiers on Thursday, the blackest day yet for the military and a dramatic ratcheting up of tensions just three days before a crunch election.

The attacks in the eastern industrial belt near the Russian border underscored the difficulties of the embattled Kiev government in resolving a crisis that is threatening to tear the country apart.

Russia lodged a furious protest with Britain on Thursday after Prince Charles reportedly compared President Vladimir Putin to Adolf Hitler, with Moscow saying the comments were unworthy of a king-in-waiting.

The Russian deputy ambassador to London went to the Foreign Office for urgent talks following the reported remarks by the heir to the throne during a trip to Canada.