US Secretary of State John Kerry has met with his Russian counterpart in what appeared to be less than amicable talks amid continuing tensions over Ukraine and American calls for a full probe into the murder of a prominent opposition figure in Moscow.

Neither Mr Kerry nor Russia's foreign minister Sergey Lavrov smiled or spoke substantively as they shook hands before photographers at a Geneva hotel, less than a week after Mr Kerry told Congress Russian officials have lied to his face about Moscow's role in Ukraine.

That comment drew a rebuke from the Russian foreign ministry. US officials have pointed out Mr Kerry did not specifically accuse Mr Lavrov of lying to him. They say he was referring to public statements and media reports, although Mr Lavrov is the only Russian official Mr Kerry is known to have met face-to-face in recent months.

The pair last met in February on the sidelines of an international security conference in Munich shortly before a new Ukraine ceasefire agreement was agreed upon.

Both Ukraine and the Russian-backed rebels have accused each other of violating the truce and the meeting comes as the US and its European allies contemplate additional sanctions on Russia for its actions in Ukraine.

In his congressional testimony last week, Mr Kerry said that fresh US sanctions had already been prepared and suggested they could be implemented soon.

However, US officials have said since that they will wait to see if the ceasefire and follow-on commitments to end the violence in Ukraine's east are complied with before putting the sanctions in place.

As the two men met, the Geneva-based UN human rights office said that more than 6,000 people have died in eastern Ukraine since the start of the conflict almost a year ago. It said the fighting has led to a "merciless devastation of civilian lives and infrastructure".

Hundreds of civilians and military personnel have been killed in recent weeks alone after an upswing in fighting, particularly near Donetsk airport and in the Debaltseve area, the body said in a report covering the period from December to February. The strategic railroad town of Debaltseve was captured from Ukrainian government forces last month by pro-Russian separatists.

While Russia denies its troops are fighting in Ukraine, the UN cited "credible reports (that) indicate a continuing flow of heavy weaponry and foreign fighters" from Russia.

Mr Kerry and Mr Lavrov each addressed the UN Human Rights Council later and a senior US official said they had met again briefly there to continue their earlier discussion.

The Kerry-Lavrov meeting also followed Friday's murder near the Kremlin of Russian opposition leader Boris Nemtsov.

Mr Kerry and other US and Western officials have called on Russia to conduct a prompt, thorough, transparent and credible investigation into the killing.

Russian president Vladimir Putin has said he will take personal charge of the probe.

US officials travelling with Mr Kerry would not say what they thought of Mr Putin's move and said they would withhold comment until the investigation is complete.