The United States said on Thursday it was concerned about reports that Russia is moving more military equipment into Syria to bolster President Bashar al-Assad, with a truce in tatters and peace talks in meltdown.
A U.S. official separately said Russia has been repositioning artillery to northern Syria - a move that may suggest the Syrian government and its allies are preparing another assault on the divided city of Aleppo.
The arrival of Russian reinforcements would risk driving the war into an even higher gear after the effective collapse of the truce and U.N.-led peace talks in Geneva aimed at ending a five-year war that has killed at least 250,000 people.
Russia’s defense ministry did not immediately respond to a Reuters request for comment.
U.N. special envoy Staffan de Mistura will on Friday assess whether Geneva talks can go on with the main opposition negotiators refusing to participate and combatants accusing each other of breaking the six-week-old ceasefire.
The opposition this week urged more military support for rebels after declaring a truce was over and that talks would not re-start until the government stopped committing "massacres".
The talks aim to halt a conflict that has allowed for the rise of the Islamic State group, sucked in regional and major powers and created the world's worst refugee crisis.
With talks on life support, all members of the main Syrian opposition will leave Geneva by Friday, a source close to the High Negotiations Committee and a Western diplomat said.
"I'm saddened and believe it's a mistake," the diplomat said. "It will be very difficult to find a pretext for them to return given the situation on the ground and now the regime knows that a bombing will ensure they stay away," he said, referring to an air strike this week that killed dozens.
Both sides are far apart and Syrian government forces have been boosted on the battlefield by Russia's firepower.
Rebels vow to fight on
France, which accused the government of rushing "headlong" into violence and showing its refusal to negotiate a political solution, said it would consider with other European powers and the United States the idea of convening a ministerial meeting of major powers in the next two weeks to work out what to do.
"If the regime insists on stubbornness, obstruction and rejection of international resolutions, we will continue our revolution," Abdullah Othman, head of the politburo of the Levant Front rebel fighting group, told Reuters.
"Our only option is to realize the revolution's goals." In March 2011, pro-democracy protests in the southern Syrian city of Deraa were crushed. This triggered nationwide demonstrations that ignited into widespread unrest and civil war.
The multi-sided conflict splintered Syria into a patchwork of areas controlled by the government, an array of rebel groups, Islamic State, and the well-organized Kurdish YPG militia.
Far from the main frontlines between government forces and rebels in western Syria, Kurdish groups, meanwhile, fought one of their most serious battles yet with government forces in the northeast, routing pro-Damascus militiamen in Qamishli.
It was a rare example of confrontation between sides that have mostly left each other to their own devices since the start of the conflict in 2011, and underlined growing Kurdish power that has alarmed neighbouring Turkey. Syrian government officials could not be reached for comment.
With violence escalating, Syria's fragile peace talks might not resume for at least a year if they are abandoned, one senior Western diplomat warned. De Mistura, who has come closer than any mediator so far in bringing the warring sides to peace talks, would not be drawn on what to expect on Friday.
Government negotiators say Assad's presidency is non-negotiable. Underlining confidence in Damascus, a top Assad aide reiterated its view that local truce agreements and "destroying terrorism" were the way towards a political solution.
The opposition wants a political transition without Assad, and says the government has failed to take goodwill measures by releasing detainees and allowing enough aid into opposition-held areas besieged by the military.
Endorsed by the U.N. Security Council, the Geneva peace talks marked the most serious effort yet to resolve the war but failed to make any progress with no sign of compromise over the main issue dividing the sides: Assad's future.
The war was tilted in Assad's favor last year by Russia's intervention, supported on the ground by members of the Iranian Revolutionary Guard Corps who have been bolstered recently by the arrival of members of Iran's regular army.
"We've been concerned about reports of Russia moving materiel into Syria," Ben Rhodes, deputy national security adviser to President Barack Obama, said at a news briefing in Riyadh where Obama was at a summit with Gulf Arab leaders.
"We think it would be negative for Russia to move additional military equipment or personnel into Syria. We believe that our efforts are best focused on supporting the diplomatic process," Rhodes added.
States opposed to Assad have been channeling military support to vetted rebel groups via both Turkey and Jordan, in a program that has included military training overseen by the U.S. Central Intelligence Agency.
Battle for Aleppo
The U.S. official, who spoke on condition of anonymity, said the repositioning of Russian artillery and some forces near Aleppo followed the Syrian government's recapture of the city of Palmyra from Islamic State.
The widely violated truce began fraying some two weeks ago near Aleppo, where the Syrian army accused rebel groups of taking part in assaults by Islamists who are not covered by the ceasefire. Rebels say they were defending themselves from attacks by the army and its Shi'ite militia allies.
Aleppo is divided into areas controlled separately by the Syrian government and opposition. To the north of the city meanwhile rebels have been battling the Islamic State group, forcing more people to flee.
Heavy air strikes have also resumed in opposition-held areas of Homs, with new battles also erupting in Latakia province.
The Qamishli fighting erupted on Wednesday. The city near the border with Turkey is mostly controlled by the Kurdish YPG militia, with the government still controlling the airport and a small area in the city.
A Reuters witness said a gun battle at a prison in the city ended with the surrender of at least 40 pro-government militiamen who had been holed up inside.
The Syrian Observatory for Human Rights, a British-based group that tracks the war, said the fighting began on Wednesday when the Kurdish internal security forces, called the Asayish, stopped a car carrying an officer of a militia that operates under the control of the Syrian army.