Arab foreign ministers have denounced "crimes against humanity" in Syria, calling on the government to stop the violence immediately, as opposition fighters came under fire on multiple fronts.

Opposition forces announced plans to reform and stem the proliferation of militias in the hope of winning support from the international community which has been reluctant to arm them.

Assad himself came under renewed diplomatic fire from Turkish Prime Minister Recep Tayyip Erdogan, who said Syria had become a "terrorist state", and from Egyptian President Mohamed Morsi, who told him to go.

At a meeting in Cairo, Arab foreign ministers condemned "the pursuit of violence, killings and ugly crimes carried out by the Syrian authorities and their shabiha militias against Syrian civilians".

"The crimes and massacres being carried out are crimes against humanity," said the ministers.

"The UN Security Council must take all the necessary measures to bring to international justice all those responsible for these crimes," the added.

The ministers also denounced Syrian authorities for unleashing heavy weapons against residential areas.

At least 26,000 people have been killed since the uprising against the Damascus regime erupted in March 2011, according to the Syrian Observatory for Human Rights.

In its latest toll, the watchdog said at least 90 people were killed nationwide on Wednesday alone, most of them civilians.

Forces loyal to the embattled president again trained their heavy weapons on the second city Aleppo which has been the target of a five-week-old government offensive aiming to dislodge the opposition fighters who took over swathes of the commercial capital in July.

Relentless bombardments and food shortages have been reported in zones held by opposition, a rag-tag army of military defectors and civilians who have taken up weapons.

An opposition general said on Wednesday that the Free Syrian Army would soon adopt changes aimed at overcoming divisions and addressing the growing number of militias fighting on its behalf.

Following talks due to end in around 10 days, the FSA would go by the name of the Syrian National Army, General Mustafa al-Sheikh, head of the military council grouping opposition chiefs, told AFP.

"After a long period, we must restructure the army because we fear the proliferation of militias in Syria and want to preserve the country's future," he said.

He said reforms were key to winning the support of the international community which has so far been reluctant to arm the opposition "on the grounds that the (FSA) is not a real institution".

Having failed to persuade the international community to impose a no-fly zone, the FSA has increasingly targeted airports used by regime attack helicopters and warplanes.

Several blasts were heard in the Damascus district of Jubar as it came under heavy bombardment, and explosions were also heard in Yalda just south of the city, the watchdog said.

In the central city of Homs, the opposition bastion of Khaldiyeh came under fierce mortar fire, and three children were killed when regime forces bombarded the Ariha area in Homs province.

UN and Arab League peace envoy Lakhdar Brahimi said on Tuesday that the death toll in the country was "staggering" and the destruction "catastrophic".

Turkey's Erdogan, who turned against Assad when the Syrian leader resorted to force against unarmed protesters, used his strongest language yet against his erstwhile ally.

"The regime in Syria has become a terrorist state," Erdogan said. "Syria is not an ordinary country to us. We do not have the luxury to remain indifferent to what's happening there."

Assad also came under fresh attack from Egypt's president.

"I tell the Syrian regime 'there is still a chance to end the bloodshed'. Now is the time for change... no time to be wasted talking about reform," Morsi told the Arab League ministers in Cairo.

He urged Assad to "take lessons from recent history" and step aside, in reference to Arab Spring revolts that overthrew the dictators of Tunisia, Egypt and Libya.

Less stridently, China said it supported a political transition in Syria and defended its record during a visit by US Secretary of State Hillary Clinton.

Foreign Minister Yang Jiechi called for all sides to end the fighting, telling a joint news conference with Clinton: "Let me emphasise that China is not partial to any individual or any party."

Clinton reiterated she was "disappointed" by Chinese and Russian vetoes of UN resolutions that would have threatened action against Assad to end the spiralling bloodshed.

The United States, meanwhile, unveiled $21 million in new aid to help those caught up in the conflict, boosting its humanitarian funding to more than $100 million.

Washington also urged Iraq to ensure that Iranian planes flying over its territory land and face cargo inspections, amid concerns Tehran is shipping arms to the Syrian regime.

PHOTO CAPTION

Syrian regime forces gather at the entrance of Aleppo's historical citadel also known as Crac Des Chevaliers under the control of the Syrian regime army on September 4.

AFP