Syrian security forces surged overnight into the Damascus suburbs of Saqba and Hamriya, cutting off communications, firing shots and making arrests, a rights group said on Sunday.
The raid comes in defiance of demands Saturday by US President Barack Obama, British Prime Minister David Cameron and Saudi King Abdullah that the Syrian regime "immediately" halt its brutal crackdown on pro-democracy protests.
"The security forces entered Saqba and Hamriya in great numbers and launched a campaign of arrests," the Syrian Observatory for Human Rights said in a statement.
The Britain-based advocacy group added that the troops arrived in "15 military trucks, eight troop carriers and four jeeps," launching the assault at around 2:00 am (2300 GMT Saturday).
"Gunfire was heard in both suburbs," the Observatory said, adding that landline telephone lines and Internet connections were cut off during the military assault.
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In a telephone conversation on Saturday, Obama and the Saudi monarch expressed their "shared, deep concerns about the Syrian government's use of violence against its citizens," the White House said in a statement.
"They agreed that the Syrian regime's brutal campaign of violence against the Syrian people must end immediately."
In a separate phone call, Obama and British Prime Minister David Cameron also agreed on the need for an "immediate" end to the bloodshed.
They reiterated "their deep concern about the Syrian government's use of violence against civilians and their belief that the Syrian people's legitimate demands for a transition to democracy should be met," the White House said.
A spokesman for Downing Street said the two leaders "expressed horror at the brutal reaction of the Syrian regime to legitimate protests, particularly during Ramadan," the holy month in which Muslims fast from dawn to dusk.
The call came after Saudi Arabia, a Sunni Muslim regional heavyweight which had remained silent on the five-month revolt, added its voice to a chorus of criticism and recalled its ambassador from Damascus.