BEIRUT (AP) -- 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.
After 13 months of violence, Western powers and opposition leaders remain skeptical about President Bashar 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 truce, which is at the center a peace plan by special peace envoy Kofi Annan, while flouting others.
Since the cease-fire formally took effect Thursday, the military has halted random shelling and mortar attacks on rebel-held residential areas, with the exception of Homs, an opposition stronghold battered by shells and mortars for more than three weeks. Before the truce, shellings were the daily norm in many areas of Syria.
The regime has ignored other parts of Annan's plan, including that it withdraw troops from towns and cities and allow peaceful protests. The regime has maintained an intimidating presence of troops, tanks and plainclothes security agents in the streets and demanded that anti-government protesters seek permits.
"What cease-fire? There's an explosion every five to six minutes," said Yazan, a Homs-based activist, contacted by Skype.
The reported shelling in Homs was threatening the truce as U.N. observers were en route to Damascus on Sunday. Each side in the Syria conflict has accused each other of truce violations.
A spokesman for Annan said six observers were scheduled to land Sunday night and will be "on the ground in blue helmets tomorrow."
Ahmad Fawzi said they "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 the cease-fire.
But on the ground in Homs it was apparent how shaky the ceasefire is in reality.
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. Other videos showed shells whistling overhead before crashing in the midst of residential buildings and one building bursting into flames after a shell exploded.
"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 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 government warned it would hit back at "armed terrorist groups" who have "hysterically" intensified their attacks against civilian and military targets since the cease-fire went into effect and the Security Council voted to send observers to Syria.
"Authorities have a duty to safeguard the security of the nation and its citizens and will prevent these terrorist armed groups from continuing their criminal attacks," a military official quoted by the state-run news agency SANA said.
The LCC said government forces also shelled the village of Khirbet el-Jouz in the northern Idlib province, near the Turkish border. Army defectors from the Free Syrian Army are known to operate in this area.
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 Assad began more than a year ago.
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. 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.
But the reception was much more muted in places like Homs. Activist Yazan said Thursday, the day the cease-fire went into effect, was the only quiet day.
"The shelling resumed Friday and it has been escalating since then."
Associated Press writer John Heilprin in Geneva contributed to this report.