BEIRUT – Syria’s four-day-old cease-fire appeared to be quickly eroding Sunday, with regime forces firing dozens of tank shells and mortar rounds at neighborhoods in the opposition stronghold of Homs, hours before the arrival of a first team of U.N. truce monitors.

Even though the overall level of violence has dropped, escalating regime attacks over the weekend raised new doubts about President Bashar Assad’s commitment to a plan by special envoy Kofi Annan to end 13 months of violence and launch talks on Syria’s political future.

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 — Syria’s third largest city — but ignored calls to pull troops out of urban centers, apparently for fear of losing control over a country his family has ruled for 40 years. Rebel fighters have also kept up attacks, including shooting ambushes.

The international community hopes U.N. observers will be able to stabilize the cease-fire, which formally took effect Thursday. A six-member advance team of U.N. observers headed to Damascus on Sunday, a day after a unanimous U.N. Security Council vote approved such a mission. A larger team of 250 observers requires more negotiations between the U.N. and the Syrian government next week.

U.N. Secretary-General Ban Ki-moon expressed serious concern at the Syrian government’s 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. “I urge again in the strongest possible terms that this cessation of violence must be kept.”

Ban said he had in-depth discussions Saturday with Annan in Geneva and expressed hope that once the full monitoring team is on the ground “there will be calm and stability and peace without any violence.”

With Assad seen as a reluctant participant in Annan’s plan, the observers’ success will depend on how much access they can negotiate in Syria and how quickly the team can grow to a full contingent, analysts said.

The Security Council demanded freedom of movement for the U.N. team, but the 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 visitors.

“This will be a serious cat-and-mouse game between the government and the U.N. for weeks to come,” George Lopez, a professor of peace studies at the University of Notre Dame in Indiana said of the new observer mission. Buying time is in Assad’s interest, he said.

However, a reassuring presence of monitors could also enable Syria’s opposition to return to staging mass marches, common in the early days of the anti-Assad uprising that erupted in March 2011.

In response to a violent regime crackdown on such protests, the turnout for weekly anti-regime marches has decreased. The opposition resorted more and more to armed attacks in recent months. By returning to peaceful protests, it would be able to regain some of the moral high ground it lost as the conflict became increasingly violent.

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.

The city of Homs was the main flashpoint of violence again Sunday. The city had been battered by daily regime shelling for three weeks before the cease-fire, and shelling resumed late Friday, less than 48 hours after the truce took effect, residents said.