Israeli Prime Minister Benjamin Netanyahu and Russian President Vladimir Putin have agreed on a plan to avoid "misunderstandings" in Syria amid an apparent military build-up by Moscow to support President Bashar Assad.

The two leaders reached an agreement on Monday during talks in Russia, with Israeli media reporting that the discussions involved avoiding clashes between the two militaries' jets over Syria.

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.

Ukraine's army said on Wednesday that a new nine-day truce with pro-Russian insurgents was holdings despite small skirmishes that claimed the lives of two government soldiers this week.

"A relative calm on the front is holding," Ukrainian military spokesman Andriy Lysenko told reporters.

Eight meters long and weighing 25 tonnes, a replica of the so-called Tsar Bomb, the most powerful nuclear bomb ever detonated, has gone on display for the first time in Russia, in the midst of an ongoing standoff with the West over Ukraine.

Tested in 1961 by the Soviet Union, the hydrogen bomb -- also known as the AN602 -- instilled a mix of pride and fear in retired military pilot Nikolay Krylov as he looked at the replica housed at an exhibition center near the Kremlin. 

Syrian President Bashar Assad described on Tuesday as "legitimate" the presence of fighters from Hizbullah in Syria to back his forces against anti-regime gunmen.

"The difference (between Hizbullah and foreign anti-regime fighters) is legitimacy. Who invited Hizbullah to Syria?" Assad asked. 

Three civilians have been killed in east Ukraine as government forces claimed Saturday that shelling by pro-Russian separatists hit a record high since the start of a tattered truce in February. 

Authorities in Ukrainian-controlled territory along the frontline told Agence France Presse that shelling over the past day had killed two civilians and wounded 15 more. 

President Barack Obama's administration vowed to press ahead with a half-billion-dollar mission to train Syrian opposition fighters Tuesday, despite fears that its initial failures have dented U.S. credibility. 

After the routing of a 54-strong U.S.-trained force by al-Qaida allied rebels, Pentagon spokeswoman Commander Elissa Smith told Agence France Presse there was no plan to pause or scale back the program.

Rescuers recovered eight bodies from a reservoir outside Moscow on Sunday following the collision of a helicopter and a seaplane in midair, with one other person missing and believed also killed in the crash. 

"We confirm that so far the bodies of eight dead have been found," the deputy governor of the Moscow region told the Interfax news agency.

France's defense minister said on Thursday that "several" countries had expressed an interest in buying its Mistral-class warships, after Paris refused to deliver them to Russia because of the Ukraine crisis.

"A certain number of countries -- there are several -- have made their interest known for these boats," Jean-Yves Le Drian told RTL radio.

Russian President Vladimir Putin is having a change of heart on the Kremlin's wholehearted support for Syrian leader Bashar Assad and may "give up on him" in the future, Turkish President Recep Tayyip Erdogan was quoted as saying Monday.

When asked if Putin could be persuaded not to support Assad, Erdogan said he saw his counterpart as "more positive" during a face-to-face meeting in the Azerbaijani capital of Baku in June and in subsequent telephone talks.