At least three people were killed by sniper fire in Harasta and Douma on Wednesday morning, they added. Shelling in Douma on Tuesday left eight dead.
Troops also reportedly shot at a bus in Idlib province, killing three people.
On Tuesday, UN special envoy Kofi Annan expressed concern about apparent surges in violence after visits by observers.
Activists said a number of people had been executed in the central city of Hama after the UN Supervision Mission in Syria (UNSMIS) left.
The reports cannot be independently verified.
The US permanent representative to the UN, Susan Rice, said all Security Council members wanted a more rapid deployment of observers to Syria.
Of a projected deployment of up to 300 monitors, only 13 have so far arrived. Ms Rice hoped 100 would be in Syria within a month.
Addressing Security Council diplomats on Tuesday, Mr Annan made it clear that the Syrian military had not withdrawn troops or heavy weapons from population centres, as it is required to do under the six-point peace plan he negotiated.
The UN and Arab League envoy suggested the Syrian authorities were also targeting people in areas where UNSMIS observers had visited and talked to residents.
Mr Annan said he was “particularly alarmed by reports that government troops entered Hama [on Monday] after observers departed, firing automatic weapons and killing a significant number of people”.
“If confirmed, this is totally unacceptable and reprehensible,” he added.
Reports of violence from Hama have diminished significantly since the UN announced that two members of the team would be staying there.
As Mr Annan spoke, activists reported that government forces were attacking opposition strongholds near Damascus, including Harasta and Douma to the north-east, which monitors have visited three times in three days.
“There was bombardment all night – artillery and tanks. We didn’t sleep at all. Not for a moment,” a woman who visited Douma told Reuters news agency on Wednesday.
“Most residents have gone down to live on the ground floor because most of the second and third floors have been hit.”
The Local Co-ordination Committees (LCC), an activist network, said government snipers had also been shooting at people on the streets of Douma, and that troops had carried out raids in several parts of the suburb, arresting suspected activists.
Several people were also shot by snipers in neighbouring Harasta, it added.
The LCC also published video of what it said were three people who were killed and four others who were severely wounded when security forces opened fire on a bus on the motorway which links the second city of Aleppo with Damascus.
The group put the nationwide death toll on Wednesday at 18, including six people in Idlib province, four in Harasta and Douma, and two each in Aleppo, Hama and Deraa.
Meanwhile, Lebanese security sources said the leader of the Lebanese Sunni militant group, Fatah al-Islam, had been killed in Syria while trying to plant a bomb for the rebel Free Syrian Army (FSA) in the town of Qusayr, near the central city of Homs.
Abdul Ghani Jawhar is said to have been an expert bomb-maker and masterminded attacks on Lebanese army and UN peacekeepers.
Fatah al-Islam has been linked to al-Qaeda and in 2007 its members fought Lebanese soldiers for 15 weeks for control of the Nahr al-Bared refugee camp near the city of Tripoli. At least 430 people were killed.
Source: BBC News