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.

Carney recalled that for President Barack Obama, the use of chemical weapons would be "unacceptable."

"It's important that we do whatever we can to monitor, investigate and verify any credible allegations, given the enormous consequences for the Syrian people," the White House spokesman said.

"We will also continue to monitor closely Syria's chemical weapons in coordination with friends and allies who share our concerns. We believe that Syria's chemical weapons stockpile remains under Syrian government control."

Carney added that Washington remains skeptical of claims by Damascus that the opposition has used chemical weapons.

Noting that Assad's regime had so far blocked a United Nations investigation of the claims and counter-claims, Carney said: "I think that demonstrates the lack of good faith on the Assad regime's part."

Earlier, the head of research and analysis in the Israeli army's military intelligence division, Brigadier General Itai Brun, said 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," 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," Kerry said in Brussels, where he was attending a meeting of NATO foreign ministers.

A senior US defense official who spoke on condition of anonymity told AFP that there were "several concerning reports" that the Syrian regime had employed chemical weapons.

"However, the use of chemical weapons in an environment like Syria is very difficult to confirm," the official said in an email sent to reporters.

Suspicions by Britain and France, conveyed to the United Nations, on Syria's potential use of chemical weapons were based on intelligence findings backed up with only "low confidence," the official said.

The United States would continue to review the allegations and was working closely with allies on the issue "to verify the claims," the official said.