U.S. President Barack Obama says the U.S. will "rethink" its options for dealing with Syria if it can establish that the government of President Bashar al-Assad has used chemical weapons against its people.

President Obama repeated to a news conference Tuesday - 100 days into his second term -  that the use of chemical weapons would be a "game changer," but he said he needed to get all the facts.

Speaking at the White House, Mr. Obama said as early as last year, he asked the Pentagon, military and intelligence officials to prepare for him what options might be available. But he refused to go into detail about what kind of military actions might be involved.