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.

‘We are in the process of preparing a political initiative, which will be significant and relevant, and which in our opinion will help the General Assembly to face the Syrian problems right in the eye,’ he said.