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.
Read more: BBC