U.S. Secretary of State John Kerry said Saturday that Syrian President Bashar al-Assad must step down, but not necessarily immediately upon reaching a settlement to end the country's civil war.

Speaking after talks in London with British Foreign Secretary Philip Hammond, Kerry said he was prepared to negotiate to achieve a solution but asked whether Assad was.

The United States and Russia renewed high-level contacts between their militaries on Friday to discuss how to deal with the war in Syria.

U.S. Secretary of Defense Ash Carter and Russia's Defense Minister Sergei Shoigu spoke by telephone, spokesmen from their ministries said.

Two German tourists were briefly detained in central Moscow after attempting to fly a drone over the Kremlin, a police spokesperson told AFP on Friday.

The tourists -- a father and his son -- were held by police after they launched a drone near one of the Kremlin's iconic towers. 

Moscow said Friday it would consider any request from Syrian President Bashar Assad to send troops, as Washington frets over an alleged Russian military buildup in the war-torn country.

"If there is any request then it would naturally be discussed and evaluated through bilateral contacts and dialogue," Kremlin spokesman Dmitry Peskov was quoted as saying by Russian news agency RIA Novosti. 

The White House said Thursday it was open to limited talks with Russia following Moscow's deployment of troops and heavy weapons to war-torn Syria.

White House spokesman Josh Earnest said the United States could be willing to take up a Russian offer of talks so long as they were "tactical, practical discussions."

Russia has jailed an activist from the Muslim Tatar community for three years for 'inciting separatism' after he criticized Moscow's annexation of Crimea, a regional court told AFP on Thursday.  

Rafis Kashapov, the head of the Public Tatar Center NGO, was found guilty of whipping up racial hatred and calling for actions "aimed at violating the territorial integrity of the Russian Federation," a court in the central Tatarstan region said.

Libya's coast guard said Thursday it had intercepted a Russian-flagged tanker suspected of carrying an illegal shipment of petrol and detained 12 crew members from Russia.

"We received a signal on the presence of a ship transporting an illegal cargo of petrol near Zuwara," 160 kilometers (100 miles) west of Tripoli, on Wednesday, Captain Taoufik Mohamad Assakir, operations chief for central Libya, told AFP.

The U.N. envoy for Syria discussed his peace proposals with officials in Damascus on Thursday, as Western fears grow that Russia is ramping up military support for President Bashar Assad.

Experts said Russia's steadfast backing for Assad and the growing waves of Syrians seeking refuge in the West might force Europe to abandon its goal of regime change to achieve peace.

Russia wants talks with the U.S. military to ensure their forces do not come into conflict as it continues its build-up in Syria, Secretary of State John Kerry said Wednesday.

Washington has expressed concern about Moscow's growing military support for the Syrian regime, even as it conducts its own air strikes against Islamic State rebels in the country.

Russia on Wednesday lambasted the U.S.-led coalition fighting Islamic State (IS) jihadists for achieving "very modest" results as Washington frets over an alleged military buildup by Moscow in Syria. 

The Kremlin -- one of President Bashar Assad's few remaining allies -- has been on a diplomatic drive to get the international coalition fighting IS in Iraq and Syria to coordinate with the regime in Damascus.