Syria conflict: World powers to hold ceasefire talks
Military officials from 17 countries are to meet in Geneva to discuss how to secure a cessation of hostilities in Syria, as a deadline they set expires.
Representatives of the US and Russia, which back opposing sides in the conflict, held talks earlier in an effort to agree a joint position.
The continuing fighting on the ground in Syria has dimmed hopes for a truce.
Meanwhile, UN envoy Staffan de Mistura conceded hopes of resuming peace talks on 25 February were not realistic.
But he also insisted the talks could be successful "if emergency aid continues and we get a ceasefire".
On Wednesday, the UN and Syrian Arab Red Crescent were able to deliver desperately-needed supplies of food and medicine for 80,000 people in three besieged towns around Damascus and two in Idlib province.
The UN is also planning a "high-altitude" airdrop of aid for 200,000 people trapped in government-controlled areas of the eastern city of Deir al-Zour that are surrounded by Islamic State militants.
Last week, members of the International Syria Support Group (ISSG) agreed that humanitarian access to besieged areas in Syria would be stepped up and that a taskforce would by Friday "elaborate modalities" for a cessation of hostilities.
They said it should apply to any of the warring parties other than so-called IS and al-Nusra Front, an al-Qaeda affiliate that is part of a major rebel alliance.
The military officials meeting in Geneva on Friday afternoon are expected discuss which areas of Syria will be covered and which groups should be included, according to the Associated Press. They will also seek agreement on what actions would constitute violations and appropriate responses.
Ahead of the talks, Mr de Mistura was quoted by the Swedish newspaper, Svenska Dagbladet, as saying: "The Americans and Russians must sit down and agree on a concrete plan on a cessation of hostilities."
The US, which backs the opposition to President Bashar al-Assad and is carrying out air strikes on IS positions in Syria as part of a multinational coalition, has criticised Russia's decision to continue its air campaign in support of government offensives in northern and southern Syria in the past week.
Washington suspects Moscow is delaying the start of the cessation of hostilities for as long as possible to give Mr Assad time to crush rebel forces around the divided northern city of Aleppo.
However, Russian foreign ministry spokeswoman Maria Zakharova told the BBC: "We are absolutely for and support everything which goes in the direction of a peace agreement."
There was no sign of a ceasefire taking force inside Syria on Friday.
The Syrian Observatory for Human Rights, a UK-based monitoring group that has a network of sources on the ground, reported clashes and air strikes in the western outskirts of Damascus, the countryside north of the city of Homs and in the northern provinces of Hama and Idlib.
Turkey, which opposes Mr Assad, was also continuing to shell a Syrian Kurdish militia across the border near the rebel-held town of Azaz, Turkey's state-run Anadolu news agency reported.
It comes after the Turkish government blamed the Popular Protection Units (YPG), which is aligned to the banned Kurdistan Workers' Party (PKK), for a bomb attack in Ankara on Wednesday.
YPG fighters have taken advantage of the Syrian government offensive and accompanying Russian air strikes in Aleppo province to capture several towns and an airbase from rebel groups.
Mr de Mistura said the Turkish bombardment was "complicating" the situation in Syria, and warned: "Any type of further conflict along the border of Syria has the potential to spin out of control."