LONDON (Reuters) - Saudi Foreign Minister Adel al-Jubeir said on Tuesday there was a possibility of reaching an agreement on a ceasefire in Syria within 24 hours.

Asked at a briefing with reporters in London to comment on the failure of the United States and Russia to agree a ceasefire, Al-Jubeir said he would not describe it as a failure but as a work in progress.

"There is a possibility of arriving at an understanding in the next 24 hours or so that will test Bashar al-Assad's seriousness to comply," he said.

But the minister went on to say Assad's history did not inspire optimism about implementation of any agreement.

A cessation of hostilities agreement brokered by U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov in February unraveled within weeks, with Washington accusing Assad's forces of violating the pact.

(Reporting by Estelle Shirbon, editing by Guy Faulconbridge)