AN Israeli allegation that Syrian forces had used chemical weapons against rebel fighters has surprised US officials, as inside the country rebels battled regime and Hezbollah forces.

Israel's Brigadier General Itai Brun, head of research and analysis in the army's military intelligence division, said on Tuesday that the Damascus regime was guilty of using chemical weapons against rebel fighters.

In remarks made to a security conference in Tel Aviv and also posted on the army's official Twitter feed, Brun referred to a March 19 incident in Aleppo province in which 31 people had been killed.

"The reduced pupils, the foam coming out of the mouth and other additional signs provide evidence that deadly chemical weapons have been used," he said, indicating the symptoms had been observed in photographs.

"Which chemical weapons? Apparently sarin. The regime is also using chemical weapons that neutralise and are not fatal," he added.

Sarin is a deadly and volatile nerve agent that is colourless and odourless.

In Brussels, where he was attending his first NATO foreign ministers meeting, US Secretary of State John Kerry appeared wrong-footed by the allegation.

He told journalists that following the report, he had phoned Israeli Prime Minister Benjamin Netanyahu.

"I think it is fair to say (the prime minister) was not in a position to confirm that in the conversation," Kerry said.

"I do not know yet what the facts are," he added.

In Washington, White House spokesman Jay Carney said: "We support an investigation, we're monitoring this, and we have not come to the conclusion that there has been that use" of chemical weapons.

But Carney also reiterated that for US President Barack Obama, the use of chemical weapons would be unacceptable.

Some members of the Damascus regime had expressed a willingness to use chemical weapons, he said.

On the other hand, Carney said the White House remained sceptical of any claim that the opposition had used chemical weapons.

Kerry had told foreign ministers from the 28 NATO nations that the alliance should stand prepared for threats from Syria, including the possible use of chemical weapons.

Pentagon chief Chuck Hagel also discussed the issue in talks with Jordan's Prince Faisal on the final part of his Mideast tour, officials said.

"The United States continues to assess reports of chemical weapons in Syria," Hagel spokesman George Little said in a statement.

Britain and France, in documents presented to the UN, suspect Syria has used chemical weapons and have been lobbying for a lifting of the EU embargo blocking arms supplies to the Syrian rebels.

In Syria itself, two Orthodox bishops reportedly kidnapped by rebels in the north of the country were released, a statement from a religious group said.

Bishop Yohanna Ibrahim and Bishop Boulos Yaziji, seized on Monday, were already at Saint Elias cathedral in Aleppo, the Paris-based "Oeuvre d'Orient" association said.

On the battle-front, the Syrian Observatory for Human Rights watchdog and activists said Hezbollah, which is backed by Iran, was fighting alongside Syrian troops in and around Qusayr in the central province of Homs.

A Syrian military source insisted that the capture of the rebel stronghold town of Qusayr was "just days away, at most".

Hezbollah's role in the fighting has inflamed tensions inside Lebanon, which has a policy of neutrality over the conflict. The group insists it is only acting to protect Lebanese citizens in Syrian border villages.

Lebanon's Salafist Sunni Sheikh Ahmed al-Assir has urged his followers to join the fight against the Syrian regime and Hezbollah forces in Qusayr, calling it "a religious duty".

The conflict, which began in March 2011 and has killed more than 70,000 people, has regularly spilled across the border, with two new mortar rounds hitting the eastern Lebanese region of Hermel on Tuesday.

In letters to the United Nations, meanwhile, Damascus condemned as an "act of aggression" the EU's decision on Monday to ease an oil embargo on Syria, so rebels could exploit the resources they control.

At least 134 people were killed throughout Syria on Monday, according to the Observatory, including 50 rebels, 47 civilians and 38 soldiers.