An initial team of U.N. ceasefire monitors is due to arrive in Syria on Sunday evening and will be deployed on Monday in an effort to defuse tensions and keep the peace plan on track, the spokesman for international mediator Kofi Annan said.

The six-person advance team will be joined by two dozen more observers in coming days in line with a U.N. Security Council resolution adopted on Saturday authorising the deployment of up to 30 people, Ahmad Fawzi said.

The Syrian government said however it had a right to refuse monitors depending on their nationalities. Spokeswoman and presidential adviser Bouthaina Shaaban said the government could not be responsible for the safety of the monitors unless it was involved in all steps on the ground.

Of course we are hoping that the process holds together until the observers get on the ground, Fawzi told Reuters.

Annan, joint special envoy of the United Nations and Arab League, brokered a six-point peace plan that was accepted in late March by the government of President Bashar al-Assad and the Syrian armed opposition to end 13 months of violence.

However, Syrian troops and their heavy weaponry did not withdraw from towns as required under the plan and there have been reports of violence and shelling.

The idea is for the unarmed monitors to deploy as soon as possible - provided their security is guaranteed - to start supervising truce compliance while Annan presses ahead with other steps including the start of political dialogue.

Annan was in close touch with top Syrian officials over the weekend, speaking with both foreign minister Walid al-Moualem and his deputy, Fawzi said. He will attend an Arab League ministerial meeting on Syria in Doha on Tuesday, he added.

IN BLUE HELMETS ON MONDAY

The first batch of six U.N. observers arrives tonight, they will be on the ground in blue helmets tomorrow (Monday), Fawzi said. The six, to be led by a Moroccan colonel, will arrive from New York, he said.

He declined to name the countries contributing observers from peacekeeping operations already deployed in the region.

In an interview with Reuters Television, Fawzi said: It is a small group of six people. I know many people will say what can six people do? ... We believe the mere presence of blue helmets on the ground in one or two locations which have seen conflict and tension will reduce tensions.

Additional observers will arrive from U.N. peacekeeping missions in the region and he expected the whole advance team to be in Syria as soon as possible and within a few days at most.

U.N. Secretary-General Ban Ki-moon, speaking in Geneva on Saturday night after private talks with Annan, said that he would make proposals by next Wednesday regarding the full observer mission.

We expect the Security Council to discuss and adopt a second resolution before the end of next week that will authorise the deployment of a full observer mission of 250, perhaps a few more, Fawzi said.

The mission will include civilians, political officers and human rights experts in order to observe the full implementation of the six-point plan, which includes a lot more than the cessation of hostilities.

(Additional reporting by Vincent Fribault; Editing by Alessandra Rizzo)