(AP) — Rebels seeking to topple Syrian President Bashar Assad killed three regime officers in separate attacks Tuesday around Damascus, activists and state media said, the latest violence targeting the security forces used by the government to quash dissent.

A bomb hidden in an army truck also exploded in the capital, wounding several people.

The persistent bloodshed has tarnished efforts by a U.N. team of observers to salvage a truce that started to unravel almost as soon as it began on April 12. Despite the violence, the international community still sees the peace plan as the last chance to prevent the country from falling into civil war – in part because there are no real alternatives.

U.N. monitors visited the restive Damascus suburb of Douma on Tuesday, their second visit in two days. Activist Mohammed Saeed said shelling and gunfire in the area Tuesday killed one person. Amateur videos posted online showed smoke rising into the sky after an explosion as well as tanks moving through the streets alongside soldiers firing weapons.

The observers also returned to the central city of Hama, where regime forces killed more than 30 people on Monday. The killings were apparently in retaliation for a large rally to welcome the team during a visit on Sunday.

An amateur video showed two women appealing to the team’s head, Col. Ahmed Himiche, for protection.

“They slaughtered us! Our children are gone! Burning and killing and slaughter!” one woman yells, preventing Himiche from shutting the door of the white U.N. Land Cruiser. “If you really came as observers for us, have mercy on us!”

Another video shows residents running for cover as men who appear to be soldiers walk across a nearby rooftop behind a wall of sandbags.

Team spokesman Neeraj Singh said the team also visited the city of Homs, where two observers are staying, and that two would remain in Hama.

The Britain-based Syrian Observatory for Human Rights said an intelligence officer was shot dead in the northeastern Barzeh neighbourhood of the capital Damascus. Also Tuesday, the state news service said “terrorists” shot to death a retired lieutenant colonel and his brother, a chief warrant officer, in an area southwest of the capital.

Inside the city, explosives planted in an army truck blew up as the vehicle traveled through downtown, leaving a hole in its roof and blood and shattered glass on the road. The truck’s driver and two passengers in a nearby car were injured and taken to a hospital.

The state news service said an “armed terrorist group” planted explosives under the driver’s side.

No one immediately claimed responsibility for the attacks.

The 11-person U.N. team is in Syria to observe the cease-fire and prepare for a total of 300 monitors to arrive later.

Tuesday’s attacks underlined the increasing militarization of the conflict, which began in March 2011 as peaceful protests calling for political reforms with inspiration from successful revolts in Egypt and Tunisia. The government cracked down with massive force, deploying troops, snipers and pro-government thugs against the opposition, while regime opponents armed themselves for protection and joined forces with army defectors.

The U.N. says more than 9,000 people have been killed since the Syrian conflict began.

The international community remains divided on how to stop the conflict, with the U.S. and many Western nations calling for Assad to leave power while Russia and China have stood by his regime.

All, however, have endorsed a plan by envoy Kofi Annan that calls for a cease-fire that was to have gone into effect on April 12 to allow for talks among all sides on a political solution to the conflict.

Despite broad backing, the plan has been deeply troubled. The Syrian government has not withdrawn its troops from populated areas or allowed media access, and its troops have shelled opposition areas. Armed rebels, too, have continued to attack military convoys and checkpoints.

The regime cites such attacks in arguing that the uprising is the work of armed terrorists acting out a foreign conspiracy.

Saeed, the Douma activist, said via Skype that the observers arrived with a continent of Syrian security, drove around town and left. Shooting started soon after.

“The news here is very bad,” he said. “There are tanks and shelling and gunfire and the army has prevented fire engines from entering.”

Activist claims and videos could not be independently verified. The Syrian government rarely comments on events inside the country and has barred most media from working in the country.

Also Tuesday, the U.N. food agency said it would deliver aid to 500,000 people in Syria, a tenfold increase since December, but warned around 1 million more people in the country still don’t have enough to eat.

The World Food Program said that by the end of the month, it will provide food for 250,000 people with help from the Syrian Arab Red Crescent. It also said it would quickly double that “in the coming weeks,” focusing on the cities of Homs, Hama, Idlib and Damascus.

The U.N. says 1.4 million people in Syria struggled to feed themselves even before the start of the conflict.