Syrian troops have stormed and shelled districts in a suburb of the capital Damascus, activists said.
The attacks came a day after the Security Council voted to expand the number of UN truce monitors to 300 members in the hope of salvaging an international peace plan marred by continued fighting between the military and opposition rebels.
An eight-member team is already on the ground in Syria, and has visited flashpoints of the 13-month-long conflict since Thursday. Fighting generally stops when they are present, but there has been a steady stream of reports of violence from areas where they have not yet gone.
Douma-based activist Mohammed Saeed said two people were killed by indiscriminate firing in the sprawling district, the scene of intense clashes between rebels and security forces before a ceasefire went into effect more than a week ago.
The Syrian Observatory for Human Rights, a Britain-based opposition group with a network of activists on the ground, confirmed the deaths.
It reported that a third person was killed overnight in the village of Hteita outside Damascus when troops opened fire from a checkpoint.
It was not immediately clear what prompted the attack on Douma. Mr Saeed said loud explosions that shook the city early on Sunday caused panic among residents, some of whom used mosque loudspeakers to urge people to take cover in basements and in lower floors of apartment buildings.
"This UN observers thing is a big joke," Mr Saeed said. "Shelling stops and tanks are hidden when they visit somewhere, and when they leave, shelling resumes."
His comments reflect widespread lack of faith among many Syrians in the UN peace plan. More than 9,000 people have been killed since March 2011 when the uprising against President Bashar Assad began, according to the UN
The Security Council approved a resolution on Saturday expanding the UN ceasefire observer mission from 30 to 300 members, initially for 90 days. The expanded force is meant to shore up a ceasefire that officially took effect 10 days ago, but has failed to halt violence.