A car bombing in Damascus on Sunday killed at least nine members of the Syrian security forces, a watchdog said, as regime warplanes bombarded the embattled town of Qusayr, a rebel holdout.

Syria's foreign minister, meanwhile, told UN chief Ban Ki-moon that the Red Cross would have access to civilians trapped in the town only after the end of military operations.

And France's foreign minister said a conference to find a political solution to the conflict could be delayed from June to July.

The Syrian Observatory for Human Rights said the suicide car bomb, in the east of the capital, appeared to have been carried out by the extremist Al-Nusra Front, which is allied with Al-Qaeda, although there was no immediate confirmation.

"At least nine regime forces were killed in the explosion of a car bomb near a police station in the Jubar neighbourhood," Observatory director Rami Abdel Rahman told AFP.

Syrian state news agency SANA reported "10 citizens were wounded when a car driven by a terrorist exploded in Jubar", but gave no information on deaths in the attack.

It said five of the injured were seriously wounded and reported clashes between rebels and regime forces.

In the central province of Homs, regime aircraft carried out a wave of air raids against the northern part of Qusayr and the outskirts of the strategic town, the Observatory said.

"Warplanes carried out multiple raids against the northern part of the city and the area between Dabaa and Qusayr," the Observatory said, adding there were no immediate details on injuries or deaths.

A day earlier, international aid groups called for the evacuation of civilians trapped in the town, where regime forces launched an assault two weeks ago.

But Foreign Minister Walid Muallem told Ban in a phone call Sunday that the Red Cross would not be allowed to enter until the assault is over.

"Syrian authorities will allow the Red Cross in cooperation with the Syrian Red Crescent access to the area immediately after the end of military operations," SANA quoted him as saying.

The Observatory said the regime has continued to bolster its forces in Qusayr, key to the regime and the rebels alike as it links Damascus to the coast, and is near the Lebanese border, providing a key rebel conduit for weapons and fighters.

On Saturday, Ban and international aid organisations expressed concern about civilians trapped in the town in central Homs province, and between 1,000-1,500 injured residents still in Qusayr.

But Muallem voiced "surprise" at the concern over Qusayr, SANA reported, "given that no one expressed this concern when terrorists took control of the city and the surrounding area."

Elsewhere in Homs province, the Observatory said at least 28 rebels were on killed on Saturday night in an attempt to seize control of an Alawite village, while six soldiers were killed in a rebel assault on a checkpoint nearby.

At least 144 people were killed throughout Syria on Saturday, the group said.

The international community has pinned its hopes for resolving the conflict peacefully on a US-Russian initiative to hold a conference that had been mooted for June in Geneva.

But French Foreign Minister Laurent Fabius said on Sunday that it could be delayed.

"'Geneva 2' is in my opinion a last-chance conference. I hope it will take place, I think it could take place in July," he said.

The fighting in Syria has increasingly drawn in neighbouring Lebanon, despite Beirut's official neutrality on the conflict.

Members of the powerful Lebanese Shiite group Hezbollah, a staunch ally of President Bashar al-Assad's regime, are fighting alongside government troops.

And some members of Lebanon's Sunni Muslim community have also crossed the border to fight alongside the Sunni-led rebels, encouraged by clerics including the influential Qatar-based Yusuf al-Qaradawi.

On Sunday, a security source said Hezbollah fighters and Syrian rebels clashed just inside Syria along the Lebanese border, leaving one Hezbollah member dead.

The fighting also spilled into Lebanon, with two rockets fired from Syria landing in the northeastern Hermel region, a Hezbollah stronghold.

Lebanon's President Michel Sleiman, meanwhile, said his country would file an "urgent complaint" to the United Nations after Israeli warplanes violated Lebanese airspace.

AFP correspondents across Lebanon, including Beirut, reported hearing the planes flying overhead.