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

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

And in Lebanon, two rockets fired from Syria struck a border area and Israeli warplanes could be heard flying low over several parts of the country.

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.

The Observatory said fierce clashes between rebels and regime forces were ongoing in Jubar, a contested neighbourhood which has been targeted by regime air strikes and mortar attacks for several months.

Elsewhere in the capital, the Observatory said, regime forces shelled the Al-Hajar al-Aswad, Assali and Qaddam neighbourhoods in the south and southwest of the city overnight.

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 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 details yet 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.

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

Abdel Rahman said around 1,000 wounded people were trapped inside Qusayr, but UN officials suggested the figure could be even higher.

"We understand there may also be as many as 1,500 wounded people in urgent need of immediate evacuation for emergency medical treatment, and that the general situation in Al-Qusayr is desperate," said a joint statement by UN humanitarian chief Valerie Amos and UN human rights commissioner Navi Pillay.

"We urge the parties to agree to an immediate ceasefire to allow humanitarian agencies to evacuate the wounded and provide life-saving treatment and supplies."

UN chief Ban Ki-moon also urged "all sides to do their utmost to avoid civilian casualties," and to help evacuate the wounded.

The international community has pinned its hopes for resolving the conflict peacefully on 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 long the Lebanese border, leaving one Hezbollah member dead.

The fighting also spilled into Lebanon, with two rockets fired from Syria landed 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.