U.S. President Barack Obama has urged his Russian counterpart Vladimir Putin to end air strikes against Syrian opposition forces, the White House said Sunday.

In a phone call with Putin Saturday, Obama stressed the need to quickly get humanitarian aid to besieged areas and initiating the cessation of hostilities across the war-wracked country, it said.

Russia must change its military targeting as it backs the Syrian regime, U.S. Secretary of State John Kerry said Saturday, as world powers seek a cessation of hostilities in the country within a week.

"To date, the vast majority of Russia's attacks have been against legitimate opposition groups," Kerry said of Moscow's air strikes in support of President Bashar al-Assad's forces. 

Sanctions against Russia will continue until it implements all aspects of the Ukraine peace agreement reached in Minsk last year, U.S. Secretary of State John Kerry said Saturday.

"Russia has a simple choice: fully implement Minsk or continue to face economically damaging sanctions," he said at the Munich Security Conference in Germany.

Russia has dispatched a new ship armed with cruise missiles to the Mediterranean, the navy announced Saturday, as reports said it is bound for Syria.

The Zelyony Dol, a patrol ship armed with Kalibr cruise missiles that only joined the Black Sea fleet in December, departed for the Mediterranean, the Black Sea fleet said in a statement.

The world has plunged into a "new Cold War", the Russian premier said Saturday, as East-West tensions over Ukraine and Syria took center-stage at a gathering of world leaders in Germany.

"We have slid into a new period of Cold War," Prime Minister Dmitry Medvedev told the audience at the Munich Security Conference.

Crimean Tatars, a minority Muslim group opposed to Russian annexation, on Friday condemned a new wave of repressions after Russian security forces raided homes and charged four with terrorist offences.

"A new wave of repressions has started in Crimea against Crimean Tatars," the community's governing body, the Mejlis, said in a statement.

Syrian Kurdish separatists on Wednesday opened a representation in Moscow amid a push by the Kremlin to have them included in Syria peace talks despite Turkey's objections.

"This is a historical moment for the Kurdish people," Merab Shamoyev, chairman of the International Union of Kurdish Public Associations, said at the ceremony in an industrial neighborhood in southeast Moscow.

The Kremlin on Wednesday downplayed a cake-throwing attack on opposition leader Mikhail Kasyanov and denied it was linked to a threat last week by Chechen leader Ramzan Kadyrov.

Late Tuesday, two men approached Kasyanov in a central Moscow restaurant and "shouted threats" before throwing a cake at him, the Parnas party leader said in a statement.

Russia has detained seven alleged members of the Islamic State group who wanted to stage terror attacks in Moscow and Saint Petersburg, authorities said on Monday.

The group was held on Sunday in the Urals city of Yekaterinburg and included Russians and Central Asian nationals, while its leader came from Turkey, the FSB security service said in a statement.

Up to 20,000 Syrians were stranded on the Turkish border Friday after fleeing a major Russian-backed regime offensive near Aleppo where a new humanitarian disaster appeared to be unfolding.

Tens of thousands of civilians have joined an exodus to escape fierce fighting involving government forces who severed the rebels' main supply route into Syria's second city.