BEIRUT— Rebels seeking to topple Syrian President Bashar Assad's regime launched three separate attacks on his security forces around Damascus on Tuesday, killing two ranking officers and rocking the capital with a booby-trapped car, activists and state media said.

The attacks took place as a U.N. team observing Syria's violence-ridden truce was visiting another area near the capital, the restive suburb of Douma. Activists and amateur videos reported shelling and gunfire in that area Tuesday, just a day after 55 people were killed across Syria — most of them in a city the observers had recently visited.

Tuesday's attacks underline the increasing militarization of the 13-month-old conflict and show the effort by Assad's opponents to chip away at the security services he relies upon to quash dissent.

The Britain-based Syrian Observatory for Human Rights said one intelligence officer was killed in the capital's Barzeh neighborhood but gave no information on how he died.

Separately, an army truck blew up as it was driving through downtown Damascus. The blast in Marjah Square near the Iranian Cultural Center left 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.

Security officials at the scene said the truck driver did not appear to be implicated in the blast, suggesting the explosives had been planted on the vehicle. The officials spoke on condition of anonymity because they were not authorized to talk to the media.

The Syrian government did not immediately comment on those attacks.

The state news service, however, said "terrorists" killed a retired lieutenant colonel and his brother in a Damascus suburb in a third attack.

No one immediately claimed responsibility for the attacks.

The U.N. says more than 9,000 people have been killed since the Syrian conflict began in March 2011 with mostly peaceful protests calling for political reforms. The government brutally cracked down, deploying troops, snipers and pro-government thugs to quash dissent, while many in the opposition have armed themselves for protection.

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 Damascus.

All, however, have endorsed a six-point plan by envoy Kofi Annan that calls for a cease-fire to allow for talks between all sides on a political solution to the conflict.

Despite broad backing, the plan has been deeply troubled since the cease-fire was to go into effect on April 12. 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 to weaken the country.

An 11-person monitoring team is currently in Syria to observe the cease-fire and prepare for a total team of 300 monitors to arrive later. But some areas that have welcomed the team with anti-government demonstrations have faced apparent retribution from the regime.

On Monday, more than 30 people were killed in the central city of Hama, just one day after the monitoring team visited the city.

One amateur video posted online showed blood in the streets. Another showed 15 bodies laid out in rows and wrapped in white cloth. The Observatory said 55 people were killed across Syria on Monday.

Syria's state news agency said U.N. observers were visiting the restive suburb of Douma Tuesday, their second visit in two days.

A Douma activist, Mohammed Saeed, said via Skype that he had heard the observers arrived but had no information on their visit.

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

Amateur videos posted online Tuesday showed an explosion in the area and smoke rising into the sky as well as men and young boys dashing for cover as gunfire is heard nearby. Activist claims and videos could not be independently verified as the Syrian government 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 there are still around a million more people in the country who 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 the 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.

(AP)