Syrian warships and security forces killed 10 people in an assault Sunday on the port city of Latakia, activists said, even as world leaders demanded an immediate end to the crushing of anti-regime protests.
Security forces also surged into the Damascus suburbs of Saqba and Hamriya overnight, cutting off communications, firing shots and making arrests, said the Syrian Observatory for Human Rights.
"Warships are attacking Latakia and explosions have been heard in several districts," the group said, adding that the main target was Ramleh suburb of the eastern Mediterranean port city.
Ten people were killed and 25 others seriously wounded, it said, revising an earlier casualty toll.
On Saturday, the military killed at least two more people and wounded 15 also in the Ramleh area of southern Latakia, a nerve centre of anti-regime protests, according to the advocacy group.
"Large numbers of residents, especially women and children" have fled Ramleh, the scene of mass protests calling for the fall of President Bashar al-Assad's regime, it said.
The Syrian Observatory said landline telephones and Internet connections with the port city were cut.
Around the capital, "security forces entered Saqba and Hamriya in great numbers and launched a campaign of arrests," said the Britain-based group.
It said 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 severed during the military assault.
In a telephone conversation on Saturday, US President Barack Obama and Saudi King Abdullah 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 called for for an "immediate" end to the bloodshed which has raged since protests broke out in mid-March.
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.
The violence has cost more than 2,150 lives, including around 400 members of the security forces, according to rights activists. Syrian authorities have blamed the bloodshed on armed gangs and Islamist militants.
Turkey, which shares a border with Syria and has a large Sunni population, has also expressed growing impatience with Assad's scorched earth policy, as has Russia, a longtime Syrian ally.
Washington has steadily ratcheted up the pressure on Damascus, imposing new sanctions and saying Assad has lost all legitimacy, but has so far stopped short of openly calling for him to step down.
Syrian troops backed by tanks have struggled to crush the revolt since pro-democracy protests turned into a full-scale uprising, despite repeated calls for restraint from world leaders.
The UN Security Council is due to hold a special meeting on Thursday to discuss human rights and the humanitarian emergency in Syria.