The United States put Syrian President Bashar al-Assad on notice on Monday, August 26 that it believes he was responsible for using chemical weapons against civilians last week in what Secretary of State John Kerry called a "moral obscenity," Reuters said.

"President (Barack) Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people," Kerry said in the most forceful U.S. reaction yet to the August 21 attack.

Speaking after UN chemical weapons experts came under sniper fire on their way to investigate the scene of the attack, White House spokesman Jay Carney said the use of chemical weapons was undeniable and "there is very little doubt in our mind that the Syrian regime is culpable." Kerry said Obama was consulting with allies before he decides on how to respond.

Military chiefs from the United States and its European and Middle Eastern allies met in Jordan for what could be a council of war, should they decide to punish Assad, who has denied using chemical weapons and blamed rebels for staging such attacks.

The Washington Post cited senior administration officials as saying Obama is weighing a military strike against Syria that would be of limited scope and duration, while keeping the United States out of deeper involvement in that country's civil war.

U.S. warships armed with cruise missiles are already positioned in the Mediterranean.