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 cease-fire 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. Saeed said loud explosions that shook the city early 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," Saeed said. "Shelling stops and tanks are hidden when they visit somewhere, and when they leave, shelling resumes."
His comments reflects 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 Saturday expanding the UN cease-fire observer mission from 30 to 300 members, initially for 90 days. The expanded force is meant to shore up a cease-fire that officially took effect 10 days ago, but has failed to halt violence.
UN chief Ban Ki-moon has accused Assad of violating the truce, and said Saturday that "the gross violations of the fundamental rights of the Syrian people must stop at once." Rebel fighters have also kept up attacks.
The eight-member advance team has visited the Damascus suburb of Arbeen, the southern province of Daraa, and the battered opposition stronghold of Homs. The monitors have not visited Douma yet.
Five monitors who toured Homs Saturday encountered unusually calm streets after weeks of shelling, and activists said it was the first quiet day in months. Two observers stayed behind in Homs to keep monitoring the city, after the rest of the team left that evening.
Amateur video posted on the Internet showed the observers, protected only by bright blue helmets and bulletproof vests, walking through rubble-strewn deserted streets lined by gutted apartments buildings. They were thronged by residents clamoring for foreign military help to oust Assad.
In one video, two monitors are seen sitting in a room listening to a Syrian man asking them to stay in Homs.
"We want you to stay, please stay ... When you come, shelling stops, killing stops. It's our blood," the man says as an observer nods his head.
The truce and the observer mission are part of special envoy Kofi Annan's plan for ending 13 months of violence and launching talks between Assad and those trying to oust him. Syria's opposition and its Western supporters suspect Assad is largely paying lip service to the cease-fire since full compliance could quickly sweep him from power.