GENEVA - Talks between senior Russian and American officials aiming for a broad ceasefire in Syria are likely to last into the weekend, as the conflict only intensifies further, the United Nations Special Envoy Staffan de Mistura said on Thursday.

In a separate negotiation, there was still hope of agreeing a regular 48-hour truce in the divided northern city of Aleppo to allow aid deliveries and medical evacuations, his humanitarian adviser Jan Egeland said.

The two negotiations reflect a desperate diplomatic effort to bring some peace amid an escalation of the five-year conflict, both in Aleppo and elsewhere in Syria, as Russian, Turkish, Iranian, Saudi and US-backed forces turn up the heat.

Military, security and diplomatic officials from the United States and Russia resumed talks in Geneva on Wednesday to give a "renewed and solid momentum to the cessation of hostilities", de Mistura said. Their talks follow a marathon meeting between US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov who failed to reach a deal last week.

"The discussions currently taking place between the US and Russia at a very high level and operational level go well beyond the 48-hour pause (in Aleppo)," de Mistura told reporters in Geneva. "We hope negotiations which have lasted too long reach an outcome. Time is short."

Egeland said only pressure from countries including Iran, Saudi Arabia and Turkey could unlock the door to aid deliveries. "It's urgent that we get an agreement. We were informed today that there are now 4,000 food rations left, that would be enough for 20,000 people and the population of east Aleppo is a quarter of a million," he said.

Last week the long-besieged rebel-held town of Daraya surrendered in a deal with Syrian government-backed forces, and de Mistura said there was clearly a Syrian government strategy to achieve similar deals in other besieged towns, which Egeland said were making "urgent pleas" to have their sieges broken.

As he missed his latest, end-August target for restarting peace talks de Mistura said he planned a new political initiative to bring the conflict to the attention of the UN General Assembly later this month, without elaborating.

Hundreds of fighters and their families were bused north into rebel-held territory in Idlib province, with other civilians transferred to government territory near Damascus for resettlement.

The Syrian army has said it is in complete control of the town, from where roughly 8,000 civilians were still due to be evacuated.

De Mistura warned there were "indications that after Daraya we may have other Darayas," pointing to worrying signs around the besieged towns of Waer and Moadamiyat al-Sham. "If Daraya was a shock, Al Waer is 75,000 people," he pointed out.

Jan Egeland, de Mistura's deputy and head of a UN-backed humanitarian taskforce for Syria, described the forced evacuation as "heartbreaking". He insisted that the devastating sieges in Syria could not be "broken by a population giving up after starvation and after bombing".

"A siege is lifted by humanitarian access and freedom of movement in and out by the civilian population," he said.

"We all failed the people of Daraya," he said, adding that the UN was receiving "urgent pleas" from besieged communities in Waer, Moadamiyat al-Sham, Madaya, Fua and Kefraya. "They all fear for their future, and we need to break the sieges," he said.

The opposition High Negotiations Committee meanwhile charged that "local truce" agreements like the one agreed in Daraya were leading to "ethnic and political cleansing".

"The Syrian regime, along with its Russian and Iranian allies, is relentlessly pursuing a malicious plan to orchestrate extensive demographic shifts across Syria," HNC head Riad Hijab said in a statement late Wednesday.

Egeland meanwhile said that humanitarian aid reached just three besieged areas last month, with two convoys reaching Waer, one reaching Harasta, and continued air-drops over Deir Ezzor, which is held by the Islamic State group.

"September must be better," he said, adding that the humanitarian taskforce had yet to receive a reply to its request to access 1.2 million people in besieged and hard-to-reach areas with aid this month.

According to the UN, more than 590,000 people live under siege in 18 areas in Syria - mostly by government forces. Nearly five million others live in areas that are difficult to access with aid. More than 290,000 people have been killed in Syria since the conflict began in March 2011.

Meanwhile,  Syrian opposition fighters have seized several areas in a rapid advance against government forces in central Hama province, prompting a wave of regime air strikes, a monitor said Thursday. At least 25 civilians, including children, were killed in the strikes early Thursday as fighting raged in parts of Hama, the Syrian Observatory for Human Rights said.

The Britain-based monitoring group said an alliance of rebel and militantt forces had launched an offensive on Monday in Hama, which is south of the opposition-held Idlib province.

The alliance, which includes the militantt Jund Al-Aqsa force, is aiming to take control of the airport in Hama, from which regime helicopters fly regular sorties against opposition fighters.

"They are about 10 kilometres (six miles) from the airport" in Hama, Syria's fourth-largest city, said Observatory director Rami Abdel Rahman, whose group relies on a broad network of sources inside Syria.

The rebels are also likely seeking to ease pressure on opposition fighters in the battleground northern city of Aleppo by distracting regime forces.

In three days the rebel alliance seized control of 13 populated areas, mainly in the north of Hama province, including the towns of Halfaya and Suran.

They were also threatening the historic Christian town of Mahrada to the west along the road.

Hama province is of vital strategic importance to President Bashar al-Assad, separating opposition forces in Idlib from Damascus to the south and the regime's coastal heartlands to the west.

Major demonstrations erupted in Hama in 2011 during the outbreak of Syria's civil conflict but were suppressed in a deadly government crackdown.

Assad's father and predecessor Hafez al-Assad brutally put down a Muslim Brotherhood uprising in Hama city in 1982, killing thousands of people.