Douma has been a hotbed of anti-Assad sentiment with massive protests taking place there late last week.
Assad has retaliated by sending in his tanks and heavy weapons:
Activists say the Sunday morning raid was launched to crush president Bashar al-Assad's opponents in Douma, an outlying suburb of the captial.
A video posted online showed columns of smoke billowing into the sky. Heavy gunfire could be heard in the background.
"Regime forces enter Douma each day, but today's assault was the largest," Damascus revolutionary council member Mohammed Saeed told AFP.
He said it was "in retaliation for huge anti-regime protests there and because it's a centre of dissent in the Damascus countryside".
The Syrian Observatory for Human Rights two civilians and four soldiers were killed in Douma.
The violence comes a day after the UN Security Council voted unanimously to dispatch 300 unarmed observers to Syria to monitor a ceasefire that went into effect on April 12.
UN envoy Kofi Annan has called the vote a "pivotal moment for the stabilisation of the country".
An advance team of UN observers toured embattled areas on Sunday, laying the ground for the new mission.
On Saturday, their visit to Homs included a stop in Baba Amr, a rebel hideout battered by a month-long army bombardment that killed hundreds, according to monitors, before it was retaken on March 1.
A YouTube video showed them meeting with activists who begged them to stay.
"Today is the first day since two months, exactly since 5 February... in Homs without shelling... without killing, without fire," one unidentified activist said in the footage.
"Because of that, we want you to stay. Please stay. This is what we want.
"When you come, shelling stops. When you come, killing stops," he told the observers, who wore blue helmets and bullet-proof vests marked UN.
It is doubtful that 300 - or 3,000 for that matter - unarmed UN observers can do much of anything if Assad wants to shell and town or move his troops and militia into a city. The observers will be totally dependent on the army for protection, food, shelter, and transportation. With that sort of control, it is hard to see the observers raising much of a stink if Assad violates the cease fire - as he has been doing since its inception almost two weeks ago.
It may not even come to that. There are no guarantees that Assad will allow 300 observers into Syria and give them unfettered access to the country. The dictator might string the UN along for a few weeks, buying time for his forces to eliminate some rebel positions, and then reneg on his promise.
That's been the pattern to date and there is nothing that Assad has done that would indicate that will change.