President Vladimir Putin on Friday ordered his forces in Syria to take tough action against any threats, speaking two weeks after Turkey shot down a Russian warplane in the war-torn country.

"I order you to act as tough as possible," he told a defense meeting. 

Russian investigators on Friday charged ex-oil tycoon and Kremlin critic Mikhail Khodorkovsky with organizing the 1998 murder of a mayor in Siberia, ratcheting up their campaign against the exiled former Yukos boss.

"As a result of investigative work, we managed to obtain new information and in light of this, it was decided on December 11, 2015, to prosecute Mikhail Khodorkovsky as a defendant for... the organization of murder," Russia's powerful Investigative Committee said in a statement.

Tensions between Washington and Moscow gave way to a fiery exchange at the State Department Thursday, with a spokesman accusing a Russian television journalist of asking "absolutely crazy" questions.

Spokesman John Kirby chided Gayane Chichakyan of state-backed RT for posing "absolutely ridiculous" questions as she persistently queried him about the U.S. position on Turkey's military deployment in Iraq during the State Department's usually cordial daily briefing.

Russian President Vladimir Putin on Wednesday invited British experts to help analyze the black box of a warplane downed by Turkey, as he discussed the Syria crisis with British Prime Minister David Cameron.

"Vladimir Putin invited British specialists to take part in decoding flight data recorders of the downed Su-24," the Kremlin said after the phone talks with Cameron.

Only 30 percent of Russian air strikes in Syria target the Islamic State group while the rest are against opposition forces not affiliated with the jihadists, a senior U.S. official said Wednesday.

"The Russian air strikes in Syria, primarily, are not attacking (IS)," Brett McGurk, U.S. President Barack Obama's special envoy for the international anti-IS coalition, told a news conference in Baghdad.

U.S. Secretary of State John Kerry said Wednesday he would head to Moscow next week to meet Russian leaders and try to build momentum for a push to end Syria's civil war.

Washington and Moscow have worked together to bring a group of regional and world powers together to forge a framework for a Syrian ceasefire and political transition.

Russian and Iranian officials addressed on Tuesday the ongoing political deadlock in Lebanon on the margins of contacts aimed at tackling the various regional conflicts, reported al-Joumhouria newspaper on Wednesday.

Russian sources told the daily that deputy Russian Foreign Minister Mikhail Bogdanov and Iranian deputy Foreign Minister Hussein Amir Abdul Lahyan discussed the Lebanese file during a brief telephone call.

Turkish Prime Minister Ahmet Davutoglu said Tuesday his government was considering retaliatory measures against Moscow, with whom it is embroiled in a bitter dispute over Ankara's shooting down of a Russian warplane.

"If we find it necessary, we will introduce retaliatory sanctions," Davutoglu told members of his ruling Justice and Development Party (AKP), adding that cabinet had discussed possible measures at a meeting on Monday.

Russian jets shelled on Monday the positions of gunmen on the eastern mountain range, reported the National News Agency.

It said that the sound of explosions could be heard in the border area with Syria on the outskirts of the northeastern border region of Arsal.

Turkish Foreign Minister Mevlut Cavusoglu on Saturday called for dialogue with Russia to overcome tensions after the downing of a Russian warplane near the Syrian border for alleged violations of Turkey's airspace.

"Of course we have different views but we need to continue to talk in order to narrow our differences," Cavusoglu was quoted as saying by the state-run Anatolia news agency.