Syrian troops shelled residential neighborhoods dominated by rebels in the central city of Homs Sunday, activists said, killing at least three people hours before the first batch of United Nations observers were to arrive in Damascus to shore up a shaky truce.

A spokesman for international envoy Kofi Annan said the group of six observers were scheduled to land that night and will be "on the ground in blue helmets tomorrow."

Ahmad Fawzi said the six "will be quickly augmented by up to 25 to 30 from the region and elsewhere." They took off shortly after the U.N. Security Council in New York voted Saturday to authorize an advance team of observers to help maintain Syria's cease-fire.

The reported shelling in Homs is threatening the truce to which President Bashar Assad and rebels fighting to topple him had agreed. Both sides accuse each other of violating the truce at the center of Annan's peace plan.

"What cease-fire? There's an explosion every five to six minutes," said Yazan, a Homs-based activist, contacted by Skype. "I can also hear the sound of a reconnaissance plane. It's flying very low."

In an amateur video posted on the Internet by activists Sunday, explosions and gunfire can be heard echoing as Khaldiyeh's skyline is engulfed in gray smoke. Homs-based activists said other districts including Bayada, Jouret el-Shayah, Qarabees and Qusour were also being bombarded.

"If you saw Homs right now you wouldn't recognize it," said Yazan, describing rubble-strewn roads and badly damaged apartment blocs. "You walk around and it's not unusual to find dead people in cars on the street," he said, giving only his first name for fear of retribution.

The British-based Syrian Observatory for Human Rights, which relies on a network of activists on the ground, said three people were killed in the shelling of Homs. The bodies of two other people were found in the Deir Baalbeh and al-Dablan districts, raising the Sunday's death toll to five.

The Local Coordination Committees activist network put the death toll at seven. It said the day started with a barrage of shells that fell at the rate of six each minute, shaking the neighborhood of Khaldiyeh for the second consecutive day.

Syrian troops shelled residential neighborhoods of Homs Saturday in the first use of heavy weapons since the cease-fire officially took effect Thursday, activists said. They say five were killed, among them Samir Shalab al-Sham, 26, a photographer and father of two who had been documenting the destruction.

He had been live-streaming the shelling on Jouret el-Shayah and Qarabees, they said, and was on the top floor of a destroyed building filming a tank when a shell struck next to him, spraying him with shrapnel.

Rebels were reported by the state media to have fired rocket-propelled grenades.

The regime restricts access of foreign observers, including journalists, making it difficult to verify reports of violence independently.

Saturday's resolution gave the 15-nation Security Council its first united front since the uprising against President Bashar Assad began 13 months ago. It called for immediate deployment of up to 30 monitors, to be followed by a larger contingent of up to 250 once the situation has stabilized.

Fawzi said the council will be asked to approve a full mission of about 250 observers _ assuming the cease-fire holds _ based on a report by the U.N. Secretary-General Ban Ki-moon next week.

Emphasizing that both sides must halt the violence that has killed more than 9,000, the council called on Syria to pull soldiers and heavy weapons out of towns and cities _ a truce provision Assad's regime has ignored. It also demanded urgent compliance with Annan's six-point plan intended to lead to talks between the regime and the opposition on Syria's political future.

The plan is widely seen as the only remaining chance for diplomacy, mainly because it has the backing of Syria allies Russia and China that shielded Assad from Security Council condemnation in the past.

Annan said in Geneva that he was "very relieved and happy" about the council vote.

Ban also welcomed the resolution.

"I will make sure that this advanced observer mission will be dispatched as soon as possible and try to make concrete proposals by eighteenth of April for an official observer mission," he said.

Western powers and opposition leaders remain skeptical about Assad's willingness to ease his tight grip on the country, ruled by his family for four decades. The regime appears to have complied with parts of the Annan plan, while flouting others.

With the exception of Homs, an opposition stronghold pounded by daily regime shelling in the three weeks leading up to the cease-fire, the military has halted random shelling and mortar attacks on rebel-held residential areas, which were the daily norm in recent weeks. However, it has maintained an intimidating presence of troops, tanks and plainclothes security agents in the streets and demanded that anti-government protesters seek permits, despite Annan's demand that peaceful gatherings be allowed.

Activist Yazan said Thursday, the day the cease-fire went into effect, was the only quiet day. "But the shelling resumed Friday and it has been escalating since then."

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Associated Press writer John Heilprin in Geneva contributed to this report.