An advance team of UN ceasefire monitors started work in Syria on Monday, seeking to ensure that both government forces and the opposition respect the terms of a truce, mediator Kofi Annan's spokesman said.
A team of six observers arrived in Damascus late on Sunday night, led by Moroccan Colonel Ahmed Himmiche.
"The mission will start with setting up operating headquarters this morning, and reaching out to the Syrian government and the opposition forces so that both sides fully understand the role of the UN observers," spokesman Fawzi Ahmad said in a statement.
"The remaining 25 are expected to arrive in the next few days," he said.
The United Nations Security Council agreed on Saturday to send the unarmed monitors to supervise the ceasefire that officially began last Thursday but has been marked by reports of violence and shelling in the flashpoint city of Homs.
On Sunday, Fawzi said he expected the Security Council to adopt a second resolution by the end of the week to authorise the deployment of a full observer mission of at least 250 monitors, including human rights experts.
One activist said the city of Homs, one of the hotbeds of opposition to Al-Assad, was bombarded on Sunday by government forces at a rate of "one shell per minute."
Other activist sources said that six people had been killed on Sunday, and four bodies had been found.
Casting further doubt on whether the ceasefire would hold, Syria said it would stop what it called "terrorist groups" from committing criminal acts, state television reported.
Annan, joint special envoy of the United Nations and Arab League, brokered the six-point peace plan in March as part of international efforts to stop 13 months of violence.
The plan calls for the start of political dialogue, the delivery of humanitarian assistance, the release of prisoners, including those involved in peaceful protests, freedom of movement for journalists to work throughout Syria.
The Security Council authorised the deployment of up to 30 observers on Saturday. This was the first resolution on Syria the 15-nation council managed to approve unanimously since the uprising erupted in March 2011.