MOSCOW/ASTANA (Reuters) - Russian-led peace talks on Syria were derailed on Tuesday as rebels backed by Turkey boycotted a third round of meetings in Kazakhstan and the Kremlin indicated there were international divisions over the process.

Russia, Syrian President Bashar al-Assad's most powerful ally, said the rebels' reasons for staying away were unconvincing and their decision came as a surprise. Describing the rebels as Turkish proxies, the Syrian government envoy said Ankara had broken "its commitments" to the Astana process.

The rebels said on Tuesday they would not attend the talks, scheduled for Tuesday and Wednesday, because of what they called Russia's unwillingness to end air strikes on rebel-held areas and its failure to get the Syrian army and Iranian-backed militia to abide by a ceasefire.

Russia has sought to revive diplomacy over Syria since its air force helped government forces defeat rebel groups in eastern Aleppo in December, Assad's biggest victory of the war.

The cooperation of Turkey, one of the main backers of rebel groups fighting in northern Syria, has been crucial to the Russian diplomatic effort, helping to broker a ceasefire in December after the rebels' Aleppo defeat.

Two previous rounds of Astana talks have sought to consolidate that ceasefire, reflecting an improvement in Russian-Turkish ties that had been strained to breaking point by the Syrian war.

Russian Foreign Minister Sergei Lavrov said the Russian Defence Ministry was in touch with Syrian rebel leaders who boycotted the talks, the Interfax news agency reported. He said Russia was dealing with the situation.

The Kremlin spokesman described the talks as hugely complex. "Sometimes the situation at these talks is really complicated because of substantial differences in approaches of various countries," Dmitry Peskov said during a conference call.

"The work continues," he said.

Syrian President Bashar al-Assad has called US forces deployed in Syria "invaders" but said he saw promise in US President Donald Trump's vow to prioritise the fight against ISIL.

About 500 US troops are in Syria in support of the campaign against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS).

The US-led coalition is currently backing a campaign by its Syrian militia allies to encircle and ultimately capture Raqqa, ISIL's base of operations in Syria.

Assad, in an interview with Chinese TV station Phoenix that was published by Syria's state television SANA on Saturday, said Trump's "rhetoric" has not resulted in "anything concrete yet".

"We have hopes that this administration in the United States is going to implement what we have heard," he said.

Asked about a deployment of US forces near the northern city of Manbij that is held by US-backed rebels, Assad said: "Any foreign troops coming to Syria without our invitation ... are invaders. We don't think this is going to help."

Assad dismissed the US-backed military campaign against ISIL in Syria as "only a few raids", and said a more comprehensive approach was needed.

The president noted the Russian-backed Syrian army was now "very close" to Raqqa city after advancing to the western banks of the Euphrates River this week - a rapid gain that has brought it to the frontier of areas held by US-backed forces.

WASHINGTON — The U.S. military's "reassurance and deterrence" mission in the Syrian city of Manbij is achieving its goal of preventing key American allies from battling one another, the Pentagon said Monday, but what's already a tense situation could become more complicated with the arrival of Russian troops and continued advances by Turkish-backed rebels.

Fewer than 100 elite Army Rangers are in Manbij to keep the peace between Syrian Kurdish forces and those loyal to Turkey, said Navy Capt. Jeff Davis, a Pentagon spokesman. Russian troops are there providing security for humanitarian convoys that have entered the war-torn city, a development he called unsurprising in light of last week's high-level talks between the senior-most military commanders from Russia, Turkey and the U.S.

The Americans and Russians have had no close interaction on the ground, Davis said. Moscow, he added, has "kept us abreast of their operations" in Manbij, but the two militaries do not coordinate in Syria. Rather, the Pentagon prefers the term "deconflict."

The dynamic in Syria is deeply complex. U.S. troops there are focused on training and assisting local forces fighting the Islamic State. Their presence has swelled to around 1,000 in recent weeks with the addition of a Marine Corps artillery unit focused on the ISIS capital of Raqqa and the Army Rangers sent to Manbij. The Russians are backing Syrian President Bashar al Assad, whose military continues to battle rebel groups seeking to oust him from power.

Meanwhile, Turkey's president has called for the liberation of Manbij from the Kurds. Ankara considers the Kurds to be terrorists linked to a militant group responsible for carrying out deadly attacks across the border.

Video posted to social media from Manbij shows the Americans' armored vehicles — a mix of Strykers, humvees and mine-resistant tactical trucks — sitting idle or traveling in convoys. "They tend to find a place to park, and look out and watch," Davis said of the Rangers' mission. "They are, for the most part, static, but they do move."