A third round of Russian-backed talks on the Syrian war opened in the Kazakh capital Astana on Tuesday without armed rebel groups, leaving little hope for a breakthrough in ending the six-year conflict. 

The talks, sponsored by regime allies Russia and Iran and rebel-backer Turkey, come as other diplomatic efforts to end the bloodshed have proved fruitless. 

Syria's conflict has killed more than 320,000 people, uprooted more than half the population and left much of the country in ruins since it erupted almost six years ago.

Syrian rebel factions will not attend a new round of negotiations with government figures in the Kazakh capital, an opposition delegation spokesman told AFP on Monday.

"Rebel groups have decided not to participate in Astana," said Osama Abu Zeid, adding that one reason for the boycott was "unfulfilled pledges related to the cessation of hostilities."

Syria's opposition is counting on the new U.S. Trump administration to play a "positive" role to end the country's six-year war, after "catastrophic" policy mistakes under President Barack Obama, an official said.

Turkey on Wednesday threatened to strike Syrian Kurdish militia forces if they do not withdraw from Manbij, a former bastion of the Islamic State (IS) group that has been taken over by predominantly Kurdish forces.

The U.N. struggled on Friday to get a new round of Syrian peace talks off the ground, but with few signs of progress as dozens more civilian deaths underlined the scale of the challenge.

The Iraqi air force struck members of the Islamic State group inside neighboring Syria on Friday, officials said, adding that the targeted militants were responsible for recent bombings in Baghdad.

The strike was announced by Prime Minister Haider al-Abadi in a statement and is believed to be the first of its kind by Iraqi jets on Syrian territory.

Turkish-backed Syrian rebels said Thursday they had fully captured the town of Al-Bab from the Islamic State group, marking a key defeat for the jihadists after weeks of heavy fighting.

As Ankara said its allies now had "near complete control" of the town, a fresh round of peace talks opened between the Syrian opposition and regime in Geneva.

Four Russian military personnel were killed and two injured when their vehicle was targeted with explosives in central Syria last week, a defense ministry statement quoted by Russian agencies said Monday.

Syria's opposition on Sunday announced its 21-member delegation, including 10 rebel representatives, for a new round of U.N.-sponsored peace talks in Geneva scheduled for February 20.