FORCES headed by President Bashar al-Assad's brother yesterday used helicopters in a new offensive against rebels in the capital, Damascus, as clashes raged in Syria's second city Aleppo, witnesses said.

The Fourth Brigade headed by Maher al-Assad was leading the assault in the Damascus district of Barzeh, triggering an exodus of residents, said the Syrian Observatory for Human Rights.

Maher al-Assad is widely seen as the military power maintaining the al-Assad family's Alawite minority rule. His role has become more crucial since Bashar al-Assad's defence and intelligence ministers, a top general and his powerful brother-in-law were killed by a bomb on Wednesday, part of a "Damascus volcano" by rebels seeking to turn the tables in a revolt inspired by Arab Spring uprisings in Tunisia, Libya and Egypt.

It was the hardest blow yet in a 17-month-old uprising.

The president has not spoken in public since the bombing.

In a further escalation of a conflict that opponents of the al-Assad regime have turned into civil war, fighting raged around the intelligence headquarters in Aleppo and in Deir al-Zor in the east. The bombardments in Damascus, Aleppo and Deir al-Zor were some of the fiercest yet and show Bashar al-Assad's determination to stay in power.

"Dozens of tanks are besieging scores of wounded people in Damascus. A medical team could not reach (the area) because of the presence of military and security forces," the Syrian Observatory for Human Rights group said, warning that lives were at risk if the siege continued.

The official Sana news agency said that government forces had "cleansed" the Qaboon district of "terrorists", the regime's term for rebel fighters.

The monitoring group estimated that more than 19000 people have been killed in the uprising.

Reports said 550 people were killed at the weekend. According to the United Nations (UN) Refugee Agency, between 10000 and 30000 Syrians had evacuated the country by crossing into Lebanon over the weekend. Overall, about 140000 Syrians are believed to have fled to Jordan.

The rebel Free Syrian Army military council's Col Abdel Jabbar al-Oqaidi said yesterday that rebels would protect civilians, notably "the minorities - Christians, Armenians, Assyrians, Kurds, Alawites, Shiites and Druze" among others.

Elsewhere in Syria rebels said they had captured a third border crossing with Turkey yesterday, Bab al-Salam north of Aleppo, while Iraqi officials said Syrian forces had regained control of one of two border crossings seized by rebels on the frontier with Iraq.

Malaysia on Saturday shut its embassy in Syria and evacuated more than 130 students and diplomats.

UN Secretary-General Ban Ki-moon said he was sending his peacekeeping chief Herve Ladsous and military adviser Gen Babacar Gaye to Syria to assess the situation.

The UN Security Council has approved a 30-day extension for a ceasefire observer mission, but Mr Ban has recommended changing its focus to pursuing prospects for a political solution - in effect accepting there is no truce to monitor.

Reuters, Sapa-AFP