WASHINGTON/BEIRUT - US President Barack Obama considered options on Saturday for a possible military strike on Syria in response to a nerve gas attack that killed hundreds as Syria sought to avert blame by saying its soldiers had found chemical weapons in rebel tunnels.
A senior UN official arrived in Damascus to seek access for inspectors to the site of last Wednesday's attack, in which opposition accounts say between 500 and well over 1,000 civilians were killed by gas fired by pro-government forces.
In the most authoritative account so far, the medical charity Medecins Sans Frontieres (MSF) said three hospitals near Damascus had reported 355 deaths in the space of three hours out of about 3,600 admissions with nerve gas-type symptoms.
The accounts and video footage of the victims - men, women and children - have heightened Western calls for a robust, US-led response after 2-1/2 years of international inaction on a conflict that has killed 100,000 people.
US military and national security advisers met Obama at the White House on Saturday to consider options for a response, the day after Washington said it was realigning forces in the Mediterranean to give him the option of attacking Syria.
Obama, long hesitant to intervene, said in a CNN interview broadcast on Friday that the United States was still gathering information about the attack.
He noted, however, that chemicals weapon use on a large scale would start "getting to some core national interests that the United States has, both in terms of us making sure that weapons of mass destruction are not proliferating, as well as needing to protect our allies, our bases in the region".
In a development that could raise pressure on him to act, American and European security sources said US and allied intelligence agencies had made a preliminary assessment that chemical weapons had been used by pro-Assad forces this week.