WASHINGTON (AFP) - The United States has not yet concluded that Syrian President Bashar al-Assad's regime used chemical weapons against rebel forces, but backs a probe into the matter, the White House said Tuesday.

"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, White House spokesman Jay Carney told reporters.

Mr Carney recalled that for President Barack Obama, the use of chemical weapons would be "unacceptable." "There are those in the Syrian government who have expressed a willingness to use chemical weapons to protect their interests and prolong the rule of the Assad regime," the spokesman said.

"We remain sceptical of any claim that the opposition used chemical weapons." Earlier, the head of research and analysis in the Israeli army's military intelligence division, Brigadier General Itai Brun, said Mr Assad was indeed guilty of using chemical weapons, likely sarin, against rebel fighters.

"One of the characteristics of the recent period is the growing use by the regime of surface-to-surface missiles, rockets and chemical weapons," Brig Gen Brun told a conference.

"To the best of our professional understanding, the regime has made use of deadly chemical weapons against the rebels in a number of incidents in the past few months," he added, in remarks quoted on the army's official Twitter feed.

Earlier Thursday, US Secretary of State John Kerry said Israeli Prime Minister Benjamin Netanyahu had not been able to confirm Syria's use of chemical weapons.

"I think it is fair to say (the prime minister) was not in a position to confirm that in the conversation," Mr Kerry said in Brussels, where he was attending a meeting of Nato foreign ministers.