UN Secretary-General Ban Ki-moon said on Monday it is the Syrian government's responsibility to guarantee freedom of movement within the country for UN observers monitoring the nation's five-day cease-fire, which appeared to be rapidly unraveling as government forces pounded the opposition stronghold of Homs with artillery shells and mortars, rebels said.

Meanwhile, the advance team of six UN observers arrived in Damascus on Sunday night, Ahmad Fawzi, spokesman for the UN-Arab League Joint Special Envoy for Syria Kofi Annan, confirmed on Monday.

Fwazi said the mission began setting up operating headquarters on Monday morning, and reaching out to the Syrian government and opposition forces so that both sides fully understand the role of the UN observers.

The remaining 25 observers are expected to arrive in the next few days.

The team was also negotiating the mission's ground rules with Syrian authorities.

Ban, speaking to reporters in Brussels, called on the Syrian government to ensure that the UN observers are not impeded in anyway in their mission.

"It is the Syrian government's responsibility to guarantee freedom of access, freedom of movement within the country," he said.

"They should be allowed to freely move to any places where they will be able to observe this cessation of violence."

Ban called the cease-fire "very fragile", but said it was essential that it hold so that an "inclusive political dialogue can continue". He said opposition forces "should also fully cooperate".

Ban added that the UN plans to increase the advance team to 30 people, all of them unarmed, and that the UN Security Council is expected to authorize a formal monitoring team of about 250 people later this week to Syria.

Fawzi said the advance team, led by Moroccan Colonel Ahmed Himmiche, met with Syrian Foreign Ministry officials to discuss ground rules, including what freedom of movement the observers would have.

The Security Council demanded full access for the UN team.

AP-Xinhua