An advance team of U.N. observers on Monday was working out with Syrian officials the ground rules for monitoring the country's 5-day old cease-fire, which appeared to be rapidly unraveling Monday as regime forces pounded the opposition stronghold of Homs with artillery shells and mortars, activists said.

Even though the overall level of violence across Syria has dropped significantly, government attacks over the weekend raised new 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.

The advance team of six U.N. monitors arrived in Damascus Sunday night. Annan's spokesman said the 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. Ahmad Fawzi said the remaining 25 observers are expected to arrive in the coming days.

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.

Two activist groups, the Local Coordination Committees and the British-based Syrian Observatory for Human Rights, said intense shelling of Homs resumed early Monday for the third consecutive day.

"Government forces trying to take control of Homs neighborhoods are pounding the districts of Khaldiyeh and Bayada with mortar fire," the Observatory said.

Both groups said two people were killed in Hama in central Syria on Monday when security forces opened fire on their car.

Western countries and the Syrian opposition are skeptical Assad will abide by Annan's six-point plan for a cease-fire and the weekend pounding of Homs along with scattered violence in other areas has reinforced those doubts.

Assad 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.

Syria's state-run newspaper Tishrin said Monday that Damascus is "satisfied" with the U.N. resolution to send observers to the country because it respects Syrian sovereignty. The paper added that the resolution says all parties were responsible for halting violence. "This is a clear cut international recognition of the crimes and assaults committed by armed groups," it said.

The international community hopes U.N. observers will be able to stabilize the cease-fire, which formally took effect Thursday. The U.N. Security Council approved the observer mission unanimously on Saturday. A larger team of 250 observers requires more negotiations between the U.N. and the Syrian government next week.

U.N. Secretary-General Bank Ki-moon expressed serious concern with the Syrian government's continued shelling of Homs and said "the whole world is watching with skeptical eyes" whether the cease-fire can be sustained.

"It is important -- absolutely important that the Syrian government should take all the measures to keep this cessation of violence," he told reporters in Brussels after meeting Belgian Prime Minister Elio Di Rupo on Sunday.

Ban said he hoped that once the full monitoring team is on the ground "there will be calm and stability and peace without any violence."

Since the cease-fire began, each side has accused the other of violations.

Syria's state-run news agency SANA has reported rebel attacks targeting checkpoints and army officers, while opposition activists said regime troops and their allied shabiha militiamen continued arrest raids and mistreatment of those in detention.

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.