Russia said Thursday it had doubled the number of troops taking part in mass drills ordered by President Vladimir Putin this week to 80,000 in a major show of strength amid tensions with the West over Ukraine.

Putin on Monday ordered drills for more than 40,000 troops in regions spanning the country, from the Arctic to the far east to the volatile southern Caucasus, and ordered nuclear bomber jets to be deployed in Crimea a year after its annexation by Moscow.

A former Chechen police officer who admitted taking part in the murder of Russian opposition activist Boris Nemtsov "likely confessed under torture", a member of the Kremlin's human rights council told AFP on Wednesday.

Zaur Dadayev allegedly confessed and was charged but has now insisted to the council's Andrei Babushkin that he is "innocent" and only made the admission under duress.

The Ukrainian girlfriend of murdered Russian opposition activist Boris Nemtsov, who was by his side when he was shot dead, has complained to police of receiving death threats, Ukrainian prosecutors said on Friday.

"Ganna Duritska... filed a statement about the threat to her life from unidentified people during her time in her parents' home" where she has stayed since her return Monday to Kiev from Moscow, said a statement from the prosecutor general. 

Russia is seeking backing from Abu Dhabi investors to build a giant airport in Cuba seen as an international gateway to Latin America, a report said Wednesday.

Talks on the proposal have been held between Moscow and Abu Dhabi state investment fund Mubadala, The National newspaper reported citing Russian Industry Minister Denis Manturov.

A plea from Ukraine for international peacekeepers to enforce a shattered ceasefire in the east ran into strident opposition Thursday from pro-Russian rebels and Moscow.

They argued the appeal, made late Wednesday by Ukrainian President Petro Poroshenko, sought to "destroy" the U.N.-backed truce agreed under European mediation last week in the Belarus capital Minsk.

The Lebanese state reportedly rejected a Russian offer to deliver to the Lebanese Army TOS-1 heavy flamethrower systems.

According to al-Akhbar newspaper published on Wednesday, Russian authorities expressed readiness, through direct and indirect channels, to deliver the multiple rocket launcher to Lebanon from a $1 billion Saudi donation.

Convoys of buses arrived in Ukraine's frontline town of Debaltseve on Friday to take besieged civilians to safety after both sides agreed a brief humanitarian truce, an Agence France Presse correspondent said.

Around 25 buses from both the Ukrainian and pro-Russian separatist sides drove into the shattered strategic town that has been the centre of fighting in recent days.

A Russian nuclear scientist has been charged with disclosing state secrets and faces up to four years in prison over an article he published in a Czech journal, his lawyer said Wednesday.

Vladimir Golubev, a former employee of the country's top nuclear weapons research and development center, is accused by the security services of disclosing state secrets after publishing an article about explosives in a Czech journal, his lawyer told AFP.

Syrian representatives of President Bashar Assad's regime and opposition figures ended a two-day round of talks in Moscow Thursday without concrete result, except an agreement to meet again.

Syria's U.N. representative and head of the government delegation in Moscow, Bashar al-Jaafari, told reporters: "We agreed with our Russian friends that discussions will continue in order to hold a (new) consultative meeting in future."

Syrian opposition figures and representatives of the regime of President Bashar Assad began talks in Moscow Wednesday but there was little hope that they would make a breakthrough in ending the country's brutal war.

The talks between opposition groups tolerated by Damascus and a Syrian delegation led by ambassador to the United Nations Bashar Jaafari came as Kurdish forces battled the Islamic State group around Kobane, after expelling them from the strategic town on the Turkish border.