A dawn cease-fire brokered by the United Nations appeared to take effect in Syria on Thursday after the government announced that it had succeeded in asserting control over the country and would stop fighting in time for the deadline.

But a Defense Ministry statement said that the government retained the right to retaliate against attacks by “armed terrorist groups,” casting into doubt its promise to halt the fighting under terms of the cease-fire proposed by U.N.-Arab League envoy Kofi Annan — the central plank in the international community’s efforts to end the bloodshed. The Syrian government has long described the 13-month uprising against President Bashar al-Assad’s rule as the work of “terrorists.”

There were no reports of violations immediately after the 6 a.m. cease-fire was to take hold, but most of the government’s attacks against opposition strongholds typically take place later in the day.

The White House urged caution on Wednesday, and activists also said they were skeptical that the cease-fire would be observed.

“The regime is going to quit shooting, but it is not going to quit arresting people and it is not going to quit torturing them,” said Omar al-Khani, a protest organizer in Damascus who uses a pseudonym. “We know we can’t trust this regime, for sure.”

Hours before the deadline for the cease-fire, government attacks continued in some opposition flash points, with Syrian troops reportedly shelling the central city of Homs and the Damascus suburb of Zabadani. Activists in Hama, in the north, said at least 20 tanks had been newly deployed in the center of the city, in violation of a clause in the cease-fire plan that calls for the withdrawal of tanks and troops from residential areas.

The Local Coordination Committees, an opposition group, said at least 91 people were killed in Wednesday’s violence, including 57 in Homs. The Syrian Revolution, another group, put total casualties at 60, but the number could not be confirmed because the government is keeping most Western journalists out of Syria.

The Defense Ministry statement, however, implied that the government believes the offensive in recent weeks against opposition centers has crushed the revolt, allowing authorities to comply with the demand to halt the fighting on Thursday.

The decision to observe the cease-fire was taken after security forces “carried out successful missions in combating criminal acts by armed terrorist groups and enforced the authority of the state on all its territories,” the official SANA news agency quoted the ministry as saying.

Annan issued a statement saying he had received a letter from Syria’s foreign minister informing him of the decision. Annan said he would continue to work with the Syrian government and the opposition to “ensure implementation” of his six-point plan, “including full compliance with Item 2,” which requires U.N. supervision of a cease-fire.

Syria has come under intense international pressure to comply with Annan’s peace plan, not only from Western powers such as the United States but also from its allies, including Russia, China and Iran. On Wednesday, Annan met with Iranian leaders in Tehran to urge them to put pressure on Damascus.

“Iran, given its special relations with Syria, can be part of the solution,” Annan said at a news conference, expressing optimism the cease-fire could work. “If every­one respects it, I think by 6 in the morning on Thursday we shall see improved conditions on the ground.”

Turkish Foreign Minister Ahmet Davutoglu scheduled a video conference late Wednesday to discuss the Syria crisis with counterparts attending a Group of Eight conference in Washington hosted by Secretary of State Hillary Rodham Clinton. In remarks opening the session, Clinton said the ministers would “look for ways that we can, together, try to bring about a peaceful resolution of the current situation and a political transition.”

Although Turkey has balked at allowing its territory to be used for a safe zone to supply and organize the Syrian opposition, a cross-border attack by Syrian government forces Monday has intensified the anger and alarm in Ankara. Turkish Prime Minister Recep Tayyip Erdogan has scheduled a Friday visit to Saudi Arabia, whose government has pledged financial aid to the Syrian opposition and called for a more robust international response.

Assuming violence continues after Thursday’s deadline, the Obama administration’s hopes of avoiding international intervention in Syria depend in large part on persuading Russia to drop its support for Assad’s government. Clinton is scheduled to meet Thursday with Russian Foreign Minister Sergei Lavrov, who is attending the G-8 gathering.

A spokesman for the Free Syrian Army, the loosely organized rebel group that claims to represent the fighters who have taken up arms against the government, said the rebels supported the cease-fire in principle. But Col. Malik Kurdi, speaking from a refugee camp in Turkey, said he doubted that it would endure.

“If the regime is committed to the cease-fire and stops its attacks, we will stop,” he said. “But we know the regime is not committed, and we will not be committed.”

“The regime is mocking the international community,” Kurdi added. “It will stop shooting for Mr. Annan for a while, but it will go back to the killing and violence and humiliation.”

Annan’s plan also calls on the Syrian government to permit peaceful demonstrations, negotiate with the opposition, allow journalists to freely visit Syria, release detainees and enable U.N. observers to monitor the process.

Abu Adnan al-Hamawi, an activist in Hama, said he thought tanks had been deployed there Wednesday to deter the anti-government demonstrations that he predicted would erupt if the government actually pulled all of its forces out of Syria’s cities, as the Annan plan requires.

“If the tanks leave, you will see 500,000 people in Assi Square,” he said, referring to the plaza where the biggest demonstrations of the uprising occurred, before protests were crushed.

Khani, the activist in Damascus, said he had heard no reports of shooting or shelling in or around the capital for the past two days. But extra checkpoints had been set up around many flash points, and additional armored vehicles were deployed in sensitive locations, apparently to make sure residents did not try to stage protests, he said.

Staff writer Karen DeYoung in Washington contributed to this report.