Syrian rebels seeking to topple president Bashar Assad have killed three regime officers in separate attacks around Damascus, 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 UN 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.
UN monitors visited the troubled Damascus suburb of Douma, their second visit in two days. Activist Mohammed Saeed said shelling and gunfire in the area killed one person.
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.
The 11-person UN team is in Syria to observe the cease-fire and prepare for a total of 300 monitors to arrive later.
The Damascus 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.
Meanwhile the UN food agency said it would deliver aid to 500,000 people in Syria, a tenfold increase since December, but warned around one million more people in the country still do not have enough to eat.
The World Food Programme 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 UN says 1.4 million people in Syria struggled to feed themselves even before the start of the conflict.