The United States on Friday demanded that the Syrian regime allow full access to UN weapons inspectors and cooperate with a probe into claims that chemical arms may have been used in the conflict.

"We support an investigation that pursues any and all credible allegations of possible use of chemical weapons," State Department spokeswoman Victoria Nuland told inspectors.

"And we demand the full cooperation of the Assad regime in letting the investigators in and letting them go wherever the trail leads them," she said, adding they should have "free and unfettered" access.

UN chief Ban Ki-moon announced on Thursday that the United Nations would investigate claims by both the Syrian regime and the opposition that the other side had unleashed chemical weapons in an attack this week.

Western nations have repeatedly warned that the deployment or use of chemical weapons, regarded as weapons of mass destruction, would be a game-changer in the two-year-old conflict.

Some US officials, including ambassador to Syria Robert Ford, have said that so far they have seen no evidence that chemical weapons were used.

US President Barack Obama said in Israel that he was investigating the reports, but he said he was "deeply skeptical" that opposition forces had used such arms.

Nuland confirmed the US would cooperate with the UN investigation while also pursuing independent inquiries, although it was complicated by the need to gather evidence on the ground in the middle of the conflict.

She refused to say definitively however if there was evidence of chemical weapons use, repeating "we need to investigate the claims. I'm not able to say one way or the other until we have full results of the investigation."

She also refused to confirm whether Washington believed that the Syrian regime was still in control of its known chemical weapons stock, saying that would stray into intelligence matters.