President Barack Obama says the U.S. has concluded that the Syrian government carried out a large-scale chemical weapons attack against civilians last week.

Obama says the U.S. has examined evidence and doesn’t believe the opposition fighting the Syrian government possessed chemical weapons or the means to deliver them.

Obama says he hasn’t made a decision about how the U.S. will respond.

The White House says it’s planning a possible military response while seeking support from international partners. But the U.S. has not yet presented concrete proof of Syrian government involvement in the attack.

Some lawmakers are calling for Obama to seek congressional approval for a military action.

(AP)