A peace plan for Syria brokered by international mediator Kofi Annan is on track despite reports of violations of the ceasefire, Annan's spokesman Ahmad Fawzi has said.

Annan will brief the United Nations Security Council on the Syria situation next Tuesday by video link from Geneva, he said.

"I would say that the Annan plan is on track and a crisis that has been going on for over a year is not going to be resolved in a day or a week," Fawzi said.

"I agree with you that there are no big signs of compliance on the ground. There are small signs of compliance."

Annan's six-point plan includes a ceasefire, deployment of observers and free access for journalists and humanitarian aid.

About 50 observers and civilian staff have been deployed in Syria, but violence has continued since the 12 April truce.

"Some heavy weapons have been withdrawn, some heavy weapons remain. Some violence has receded, some violence remains," Fawzi said. "I'm not saying that is satisfactory".

"There are signs on the ground of movement, albeit it’s slow and small. There are also signs behind the scenes you don't see because this mediation effort by definition is conducted below the radar," he said.

"However... even on days we feel there is satisfactory progress albeit it is in inches, not in feet or miles, in those days as well we are horrified by the extent of violence that we see on the ground."

The United States said on Tuesday that a new international approach may be needed if Annan's UN-backed plan fails, accusing President Bashar al-Assad of making "no effort" to implement it so far.