While Kyiv has accepted a 30-day ceasefire, Ukrainian officials are concerned that Russia would try to attach conditions, including suspending military aid.
A Kyiv official said a truce would be reached in a week if Moscow truly agreed to halt its military operations.
The UK and European Union are working on fresh military aid packages for Ukraine.
UK PM Keir Starmer reiterated Ukraine must be placed in the "strongest possible position" for lasting peace.