RIYADH/GENEVA - 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.

Fighting raged across Syria after the truce, brokered by Washington and Moscow to allow talks to take place, ended and both sides geared up for more war. Russian intervention late last year swayed the conflict in Assad's favour.

"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.

Russia's military on Thursday said it had completely demined the ancient site of Palmyra in Syria after it was recaptured by government forces from Islamic State fighters last month. "As of today the tasks to demine the ancient architectural part of Palmyra have been completed in their entirety," Yury Stavitsky, the commander of Russia's military engineers, told President Vladimir Putin in a televised video link-up from Syria.

Aid groups launched their largest aid delivery yet in war-torn Syria on Thursday after the UN evacuated hundreds of besieged residents, intensifying relief efforts even as peace talks falter. The two major operations were a rare sign of humanitarian progress in Syria, where the brutal five-year war has left 270,000 dead as a partial ceasefire hangs by a thread.

The dire humanitarian situation had even stalled UN-backed peace talks in Geneva, with the opposition walking away in frustration at sieges and attacks on civilians.

Russia has kept a considerable military presence in conflict-torn Syria to bolster the regime of President Bashar al-Assad, Nato chief Jens Stoltenberg said Thursday.

"Despite the announced partial withdrawal we see that Russia maintains a considerable military presence in support of the Assad regime in Syria," he said at a press conference in Turkey. A month ago Russian President Vladimir Putin surprised the West by ordering the "main part" of his forces to pull out.

UN special envoy Staffan de Mistura will on Friday assess whether Geneva talks can continue with the main opposition negotiator 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.

Major nations have urged both parties not to miss this chance to try to halt the five-year conflict in which more than 250,000 people have been killed but on Thursday only experts were meeting and more opposition representatives were leaving.

Syrian government negotiators say Assad's presidency is non-negotiable while the opposition says the president must step down and complains of no progress on an end to violence, humanitarian access and political detainees. Both sides remain far apart and it will be difficult to lure the opposition back to the table if fighting resumes unchecked, with the government taking advantage of Russia's firepower.

Press reports in the United States have indicated that Russia has moved more artillery into Syria, weeks after declaring a partial withdrawal of its military presence there. States opposed to Assad have been channelling military support to vetted rebel groups via both Turkey and Jordan, in a programme that has included military training overseen by the US Central Intelligence Agency.

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.

The talks in Geneva 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. Total collapse of the talks would leave a diplomatic vacuum allowing further escalation of the war that is being fuelled by rivalries between foreign powers including oil producers Iran and Saudi Arabia.

Syria's fragile peace talks might not resume for at least a year if they are abandoned now, a senior Western diplomat warned. "If we all leave Geneva, I don't see the process continuing." Russia says that its intervention in Syria consists mainly of air and rocket strikes. It says its presence on the ground is limited to a naval base at the port of Tartous, an air base at Hmeymim, in Syria's Latakia province, search-and-rescue crews to recover downed air crews, de-mining specialists, and advisers.

In the past month, it has acknowledged for the first time that it has special forces conducting operations behind enemy lines. It has previously denied having regular units and artillery on the ground.

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.

The opposition accuses the government of violating the cessation of hostilities to capture Aleppo, Syria's most populous city before the war, which has been divided between government-controlled and rebel-held zones for years.

The United Nations expressed deep concern over the fate of Syrians who have fled fighting near the northern city of Aleppo.

The Syrian government negotiator Bashar Ja'afari poured contempt on the opposition for its partial walkout, accusing it of sulking and political immaturity.

Leaving Geneva on Thursday, senior Syrian opposition negotiator Mohammad Alloush said Syrian government forces must "stop massacres" before talks can resume.

De Mistura said on Thursday there had been modest progress on humanitarian aid in Syria and Russia was pushing for a convoy to enter the besieged town of Daraya. He said in the next few days he would appoint a coordinator to handle the issue of detainees in Syria.

The envoy said prevention by the Syrian government of the passage of medical equipment into besieged areas was "worrisome" and against international law.