Washington's new Moscow envoy John Tefft swiftly took to Twitter upon arrival in Russia, saying on Friday that he was looking forward to what promises to be a tricky job.

"I am very happy to be back in Russia," Tefft, known for supporting the pro-Western aspirations of former Soviet states, said on the U.S. embassy's Twitter account in Russian. 

Interior Minister Nouhad al-Mashnouq is expected to visit Moscow on September 18 to sign a deal to buy military equipment for the Internal Security Forces, al-Akhbar newspaper reported on Monday.

Lebanon will buy the equipment from a recently announced $1 billion Saudi grant to Lebanon, it said.

A Ukrainian fighter jet has been shot down by a Russian anti-aircraft system in the east of the country, where Kiev is fighting pro-Moscow rebels, Ukraine's military said on Saturday.

Sitting beside a trench, the smoke of nearby combat visible, "Batya" looks towards the besieged town of Ilovaysk and considers the options of the pro-Kiev fighters there battling for survival.

Fierce fighting between government forces and pro-Russian rebels left dozens of civilians dead on Wednesday with artillery pounding central Donetsk as Ukrainian troops pushed on with a bloody offensive.

Deadly battles to crush the ailing rebellion appeared to intensify ahead of a fresh round of diplomacy that will see the presidents of Russia and Ukraine meet next week for the first time in months.

The White House urged Moscow on Friday to stop its "extremely dangerous and provocative" attempts to destabilize Ukraine, where tensions have soared again in the past 24 hours.

As Western countries launch each new salvo of sanctions against Russia over the Ukraine crisis, Moscow has returned fire with bans on food imports -- ostensibly for consumer protection.

Milk, cheese and onions from Ukraine, Greek peaches, Serbian prunes, Polish apples and cabbage, Spanish meat -- according to Russia's food safety regulator Rosselkhoznadzor all these products contain toxic elements, are infected with dangerous bacteria or violate other regulations.

The mayor of Moscow on Tuesday announced he had fired the head of the capital's metro over a devastating crash that killed 22 people and injured more than 200.

"What happened on the Arbatsko-Pokrovskaya line, this tragedy, has canceled out the huge amount of work that was carried out in the metro in recent years," Mayor Sergei Sobyanin told a meeting.

British Prime Minister David Cameron on Sunday told Russia's Vladimir Putin the shooting down of the MH17 passenger plane was "totally unacceptable" and appeared to be the work of Russian separatists.

"The evidence suggested that pro-Russian separatists were responsible and the Prime Minister made clear that if Russia wants to put the blame elsewhere they would need to present compelling and credible evidence," a press release about the phonecall from Downing Street said.

Russian investigators said Wednesday they had detained two metro workers over a devastating crash in the Moscow subway that killed at least 21 people, as anger mounted over the tragedy.

City authorities declared Wednesday a day of mourning after the worst accident in the metro's eight-decade history, while Russians swapped stories of how the authorities had ignored their complaints about the famed transport system.