Thousands of people took part in anti-government demonstrations across Syria on Friday, as a tenuous cease-fire unraveled further amid reports from both sides of violent attacks.

State media reported that 18 members of the security forces were killed in attacks by “armed terrorist groups” in Daraa and Hama provinces and the city of Aleppo. Activists in Hama province reported a heavy security presence and shooting directed at demonstrators, while the Local Coordination Committees described heavy shelling in the northern city of Homs. The Britain-based Syrian Observatory for Human Rights told news services that 16 civilians were killed Friday.

The numbers could not be verified, as Syria restricts journalists’ access, but the violence highlighted serious doubts in the international community about whether a peace plan proposed by U.N.-Arab League envoy Kofi Annan and agreed to by Syrian President Bashar al-Assad can ever work. The U.S. ambassador to the United Nations, Susan Rice, said in a tweet on Thursday that the cessation of armed violence in Syria, a requirement of the plan, was “incomplete — to say the least.”

France’s foreign minister, Alain Juppe, told French news media Friday that a U.N. Security Council resolution to deploy a U.N. peacekeeping mission to Syria, currently under discussion, should provide for air support and other equipment.

“We need observers on the ground, but properly equipped observers, with helicopters, that can ensure the right to protest,” he said. “It’s extremely important. The day this freedom is guaranteed, the regime will fall.”

An advance team of U.N. observers, tasked with monitoring the cease-fire, decided not to patrol on Friday, the day many opponents of the government hold rallies after noon prayers.

Earlier in the week, the team had traveled to Daraa, the southern city where the uprising began, and to the unsettled suburban areas east of the capital, Damascus, where they were surrounded by demonstrators in a tense atmosphere in which shots were fired, apparently by security forces.

On Friday, the small team chose to deal with administrative issues and plan logistics, according to U.N. officials, although the team’s leader, Col. Ahmed Himmiche, told the Reuters news that he was concerned the presence of observers might trigger an escalation of the crisis. When Arab League monitors attended Friday protests early this year, there was a surge in the numbers of people attending.

Activists speaking from Syria said they were disappointed. “They are an observer mission,” said an activist in Damascus who uses the pseudonym Hazrid. “Shouldn’t they be out there observing?” A clause in the peace plan requires the Syrian government to allow peaceful demonstrations.

“This is saying to the opposition that your peaceful protests are not necessarily something that we’re going to observe,” said Salman Shaikh of the Brookings Doha Center in Doha, Qatar. “And, to the regime, they’re trying to placate one of their biggest fears, which is that if the cease-fire is implemented we’re going to see massive protests.”

Ahmad Fawzi, Annan’s spokesman, noted that the observers had been on the ground only a few days and said he had no reason to think they would not attend demonstrations next Friday.