DAMASCUS — Syrian ships and security forces killed more than 20 people in an assault Sunday on the port city of Latakia, activists said, as world leaders demanded an immediate end to the ruthless crushing of dissent.

The Syrian Observatory for Human Rights said in a statement that at least 23 people were killed and dozens of others wounded, several of them critically.

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.

Ships are "attacking Latakia and explosions have been heard in several districts," the group said earlier, adding that the main target was the suburb of Ramel in the eastern Mediterranean port city.

Ten people were killed and 25 others seriously wounded, it said. The group said Palestinians also figured among the casualties of the assault on Ramel, which is home to Palestinian refugees in a camp.

On Saturday, the military killed at least two more people and wounded 15 also in the Ramel 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 Ramel, 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," according to 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."

Syria's human rights groups, in a joint statement, on Sunday urged the authorities to release the head of the Syrian League for the Defence of Human Rights, Abdel Karim Rihawi, who was arrested on Thursday in Damascus.

His detention "represents a violation of the international commitments undertaken by Syria," they said.

"Security forces are continuing mass arrests, in violation of the law, human rights and democratic freedoms, denying the rights of opposition figures and peaceful demonstrators," the groups said.

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 an "immediate" end to the bloodshed which has raged since protests broke out in mid-March.

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.

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.