UN-Arab League envoy Kofi Annan expects both the Syrian government and the opposition to fully implement a ceasefire agreement by April 12, his spokesman said on Thursday.

"What we expect on April 10 is that the Syrian government will have completed its withdrawal from populated centres ... and then we begin a 48-hour period during which there will be a complete cessation of all forms of violence by all parties," Ahmad Fawzi told reporters.

"So the clock starts ticking on the 10th on both sides to cease all forms of violence," he added.

The Syrian regime has told Annan that it has begun withdrawing troops from some parts of the country, said Fawzi, adding that "we are in the process of verification."

An advance team dispatched by Annan to pave the way for peacekeepers is expected to arrive in Damascus today.

They will "begin discussing with the Syrian authorities the modalities of the eventual deployment of this UN supervision and monitoring mission," said Fawzi.

The peacekeeping mission would therefore be "ready to move in as soon as we have a Security Council resolution," he added.

Meanwhile, Annan will also travel on April 11 to Tehran, where he is to seek support from the Syrian ally for his peace plan.