JERUSALEM (Reuters) - President Barack Obama said on Wednesday that the United States was investigating whether chemical weapons had been used in Syria, and that Syrian President Bashar al-Assad would be held accountable if they had been.

"We have been clear that the use of chemical weapons against the Syrian people would be a serious and tragic mistake," Obama said at a joint news conference in Jerusalem with Israeli Prime Minister Benjamin Netanyahu.

"The Assad regime must understand that they will be held accountable for the use of chemical weapons or their transfer to terrorists," he said.

(Reporting by Ari Rabinovitch; Editing by Michael Roddy)