Syrian pro-government forces celebrate on a street in the town of Sheik Miskeen on Tuesday, after retaking the strategic town from rebel forces. (AFP/Getty Images)

Western-backed Syrian rebels on Tuesday suffered one of their most significant defeats since Russia’s military intervention in Syria turned the tide of the war in favor of President Bashar al-Assad, further complicating prospects for a negotiated settlement at peace talks scheduled later this week in Geneva.

After a month-long offensive backed by Russian warplanes, government forces and allied ­militias reclaimed control of the town of Sheikh Miskeen, strategically located at a crossroads commanding a southern supply route between the Jordanian border and the Syrian capital, Damascus.

It was the latest in a string of defeats inflicted on rebel fighters in recent weeks, as Assad loyalists finally start to capitalize on nearly four months of intense Russian airstrikes that have mostly targeted the anti-Assad rebellion.

This was, however, the most conspicuous example yet of the ways in which Russia’s intervention in Syria has aided government advances against the moderate rebels backed by the United States and its allies. Sheikh Miskeen had been under the control of a coalition of moderate Free Syrian Army groups formed early in 2014 with the express purpose of streamlining the delivery of weapons and money from the United States and its allies.

A picture taken Tuesday shows spent mortar shells on the ground after Syrian pro-government forces retook the town of Sheik Miskeen. (AFP/Getty Images)

The official government news agency SANA said the “terrorists” defending the town had suffered heavy losses in the course of the battle. “Other terrorists fled away leaving their weapons and ammunition behind,” the agency said.

The definition of who counts as a terrorist in Syria lies at the heart of a dispute between Russia and the United States over how to resolve the Syrian war that has delayed the start of the peace talks. Russia has labeled all those fighting Assad as “terrorists,” while the United States and its allies draw a distinction between moderate rebels and those with extremist inclinations.

A British official said the attack revealed the “hypocrisy” of ­Russia’s military intervention in Syria, which was ostensibly launched to combat the Islamic State but has, according to U.S. officials, focused mostly on bombing the wide variety of rebels opposed to Assad’s regime.

“By continuing to support the regime in its bombardment of the moderate opposition, Russia risks damaging the already fragile process of intra-Syrian negotiations,” Gareth Bayley, the British special representative for Syria, said in a statement.

News of the fall of the town cast a cloud over a meeting of the Syrian opposition in the Saudi capital, Riyadh, to decide on whether to participate in the peace talks, scheduled to begin Friday in Geneva. The United Nations issued invitations to the talks Tuesday, after days of squabbling over the guest list between Washington and Moscow had delayed the original start date of Jan. 25.

It is still not clear, however, whether those invited will accept. The opposition said late Tuesday that before confirming its attendance it wants evidence that the government is serious about talking peace in the form of confidence-building measures such as the delivery of humanitarian aid to towns besieged by the government and a halt to airstrikes against civilians.

Russian Foreign Minister Sergei Lavrov on Tuesday reiterated the Russian view that it is “essential” to invite the most powerful Syrian Kurdish political party, the Democratic Union Party (PYD) to the talks. But the head of the PYD, Saleh Muslim, told Reuters he had not received an invitation.

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