WASHINGTON (REUTERS) - US Secretary of State John Kerry spoke to Syrian foreign Walid al-Moualem and told him the Damascus government should have let UN inspectors have access to the site of an alleged gas attack, the State Department said on Saturday.

Mr Kerry called "to make clear that if, as they claimed, the Syrian regime has nothing to hide, it should have allowed immediate and unimpeded access to the site rather than continuing to attack the affected area to block access and destroy evidence," a State Department official said.

"The secretary further emphasised ... that he had received full assurances from Free Syrian Army commanders that they would ensure the safety of UN investigators into the targeted areas," the official said.

Mr Kerry, who participated remotely in a White House meeting on a potential US response to the alleged use of chemical weapons in Syria, made a round of diplomatic calls on Saturday to his counterparts in Saudi Arabia, Jordan and Turkey, the official said.

"In all these calls, the secretary emphasised the importance of quickly determining the facts and underscored the seriousness and gravity of any chemical weapons use," the official said.