BEIRUT/AMMAN: Syrian troops backed by helicopters and tanks have struck back in Damascus against rebel fighters emboldened by a bomb attack against President Bashar al-Assad's inner circle.

Assad's forces, who also pushed into a rebel-held district in the northerly commercial hub of Aleppo on Saturday, targeted pockets of lightly armed rebels, who moved about the streets on foot and attacked security installations and roadblocks.

Residents said the sound of shelling in the capital was so intense at dusk that they were unable to distinguish it from the traditional cannon blast marking the end of the daily fast for the Muslim holy month of Ramadan.

Opposition activists said helicopters fired rockets into a neighbourhood near the southerly Sayida Zeinab district, causing dozens of casualties. They did not have any other details.

"In Damascus, people continue to search desperately for safety," the International Committee of the Red Cross (ICRC) said in a statement in Geneva.

"Humanitarian needs are growing as the situation in the city worsens and as large numbers of people flee their neighbourhoods in search of safe haven. The ICRC and the Syrian Arab Red Crescent have intensified their response to the situation."

The Syrian Observatory for Human Rights, an opposition group that monitors the violence, said 140 people, including 43 troops, had been killed across Syria on Saturday. Most of them died in the province of Homs, epicentre of the uprising.

One Western diplomat who declined to be named said Assad was moving troops to protect his capital and compared him to a doctor "abandoning the patient's limbs to save the organs".

Most shops were closed and there was only light traffic - although more than in the past few days. Some police checkpoints, abandoned earlier in the week, were manned again.

Many petrol stations were closed, having run out of fuel, and those that were open had huge lines of cars waiting to fill up. Residents reported long queues at bakeries.

The army's push in the capital occurred after rebels assassinated four of Assad's top security officials last week as part of a six-day attack in the capital that they dubbed "Damascus Volcano".

Rebels also captured three border crossings with Iraq and Turkey, and on Saturday an Iraqi security source said gunmen appeared to be taking over a fourth at Yarubiya in Syria's Kurdish northeast.

NO COMMENT FROM ASSAD

Assad, battling a 16-month-old uprising against his family's four decades of autocratic rule, has not spoken in public since the assassinations, and failed to attend funeral ceremonies for his brother-in-law and two other slain officials on Friday.

A bloody crackdown on what began as a peaceful revolt has increasingly become an armed conflict between an establishment dominated by Assad's Alawite minority, an offshoot of Shi'ite Islam, and rebels drawn largely from the Sunni majority.

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

Opposition activists in Aleppo, Syria's biggest city, said hundreds of families were fleeing residential areas on Saturday after the military swept into the Saladin district, which had been in rebel hands for two days.

Fighting was also reported in the densely-populated, poor neighbourhood of al-Sakhour.

"The sound of bombardment has been non-stop since last night. For the first time we feel Aleppo has turned into a battle zone," a housewife, who declined to be named, said by phone from the city.

REBEL BORDER CROSSING RAID

On the Iraqi-Syrian border, a security source and a separate witness said they saw gunmen in a civilian car enter the Yarubiya crossing point on the Syrian side of the frontier.

"When we contacted the Syrians there, they told us the Syrian security elements are gradually withdrawing from the place," said the security source, who works for the Iraqi customs department.

It was not immediately possible to verify the reports on the border post, but Syrian opposition activists said several towns in Syria's Kurdish northeast had passed - without a fight - into local hands in recent days as central authority eroded.

The surge in violence has trapped millions of Syrians, turned sections of Damascus into ghost areas, and sent tens of thousands of refugees fleeing to neighbouring Lebanon.

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

Diplomats said only half of the 300 unarmed observers would be needed for Ban's suggested plan, and several monitors were seen departing from Damascus on Saturday.

Speaking two days after Russia and China vetoed a resolution to impose U.N. sanctions on Assad's government, Ban called on he Security Council to "redouble efforts to forge a united way forward and exercise its collective responsibility".

"The Syrian government has manifestly failed to protect civilians and the international community has collective responsibility to live up to the U.N. Charter and act on its principles," he said.

Regional and Western powers have voiced concern the conflict might become a full-blown sectarian war that could spill across borders. But Assad's opponents remain outgunned and divided.

French Foreign Minister Laurent Fabius, speaking after contacts with the head of the Arab League and Qatar's prime minister, said all three agreed that it was time for Syria's fractured opposition to prepare to take charge of the country.

"We would like to see the rapid formation of a provisional government representing the diversity of Syrian society," said Fabius. Syria's main political opposition group, the Syrian National Council, operating in exile, has so far failed to unite Assad's disparate foes on a united political platform. AGENCIES