BEIRUT (AP) -- U.N. Secretary-General Ban Ki-moon said Monday the Syrian government is responsible for guaranteeing U.N. observers full freedom of movement to monitor the country's tenuous cease-fire, which appeared to be unraveling as regime forces pounded the opposition stronghold of Homs, activists said.
Even though the overall level of violence across Syria has dropped significantly since the truce took effect Thursday, the government's shelling of Homs over the weekend raised fresh doubts about President Bashar Assad's commitment to special envoy Kofi Annan's plan to end 13 months of violence and launch talks on the country's political future.
An advance team of six observers arrived in Damascus late Sunday and quickly set about negotiating the mission's ground rules with Syrian authorities.
Ban, speaking to reporters in Brussels, called on Assad to ensure the observers are not impeded in any way in their work.
"It is the Syrian government's responsibility to guarantee freedom of access, freedom of movement within the country," he said Monday. "They should be allowed to freely move to any places where they will be able to observe this cessation of violence."
He 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."
The U.N. plans to increase the advance team to 30 people, all of them unarmed, Ban said, adding that the Security Council is expected to authorize a formal monitoring team of about 250 people later this week.
The advance team, led by Moroccan Col. Ahmed Himmiche, met Monday with Syrian Foreign Ministry officials to discuss ground rules, including what freedom of movement the observers would have, according to Annan's spokesman, Ahmad Fawzi.
Although the Security Council has demanded full access for the U.N. team, Assad's regime could try to create obstacles. The failure of an Arab League observer mission earlier this year was blamed in part on regime restrictions imposed on the monitors, including having to travel with government minders.
Fawzi said in a statement issued in Geneva on Monday that the mission "will start with setting up operating headquarters, and reaching out to the Syrian government and the opposition forces so that both sides fully understand the role of the U.N. observers."
"We will start our mission as soon as possible and we hope it will be a success," Himmiche told The Associated Press as he left a Damascus hotel along with his team Monday morning.
The international community hopes U.N. observers will be able to stabilize the cease-fire, which formally took effect Thursday, although pockets of violence have persisted, particularly in the central cities of Hama and Homs.
Tarek Badrakhan, an activist from the battered and almost deserted Homs district of Khaldiyeh, said the regime resumed its intense bombardment of the neighborhood early Monday.
"The shelling hasn't stopped for one minute since this morning. There are buildings on fire right now," he said via Skype.
Badrakhan and other activists said the army appeared to be pushing to take control of the last rebel-held districts in Homs and was pounding Khaldiyeh from three sides. He said half of the nearby district of Bayada fell under the army's control Sunday night. Troops were trying to storm Qarabees and Jouret al-Shayah but the Free Syrian Army is repelling them, he said, referring to the army defectors fighting the government.
"We hope that the observers would come to Homs as soon as possible because if things go on like this, there won't be anything left called Homs," Badrakhan said.
Two activist groups, the Local Coordination Committees and the British-based Syrian Observatory for Human Rights, confirmed the intense shelling of Homs. They also said at least five people were killed in gunfire in the central city of Hama and four in the northern city of Idlib Monday.
The pounding of Homs and other scattered violence has further fueled skepticism among Western countries and the Syrian opposition that Assad will not abide by Annan's six-point plan for a cease-fire.
The Syrian leader accepted the truce deal at the prodding of his main ally, Russia, but his compliance has been limited. He has halted shelling of rebel-held neighborhoods, with the exception of Homs, but ignored calls to pull troops out of urban centers, apparently for fear of losing control over a country his family has ruled for four decades. Rebel fighters have also kept up attacks, including shooting ambushes.
The U.N. Security Council approved the observer mission unanimously on Saturday.
It's the first peace initiative to enjoy broad backing, including from Russia and China, who shielded the Syrian regime from Security Council censure in the past. Syrian officials said Foreign Minister Walid Moallem was headed to China for a two-day visit. Last week, Moallem met with his Russian counterpart in Moscow.
Also Monday, a Hamas official said a senior member of the Palestinian group, Mustafa Lidawi, was abducted over the weekend near Damascus. In the past, Lidawi had served as the Hamas representative in Iran and Lebanon.
Lidawi opposed a recent power-sharing agreement between the Islamic militant Hamas and its Western-backed rival, Palestinian President Mahmoud Abbas, and was seen as a supporter of Assad's regime. Until recently, Hamas' top leaders were based in Damascus, but became increasingly critical of Assad's crackdown on the uprising and decided to leave the country.
Hamas asked the Syrian authorities to try to find Lidawi, said a senior official of the group in the Hamas-run Gaza Strip, who spoke on condition of anonymity because of the sensitive nature of the contacts. Lidawi's family told Hamas officials he was abducted Saturday.
Associated Press writers Albert Aji in Damascus, John Heilprin in Geneva, Don Melvin in Brussels and Ibrahim Barzak in Gaza City, Gaza Strip contributed to this report.