president now facing some tough choices on syria. 24 hours after intelligence revealed syria has used chemical weapons against its own people. Tonight, the u.N. Is demanding syria let experts examine the evidence. Jonathan karl at the white house tonight. The president used carefully chosen words on this today. Reporter: Sure did, david. Make no mistake. This was tough talk from the president. And it cake as the white house reaffirmed that all options are on the table when it comes to syria, including military action. This video, from a syrian opposition group, purports to show the gruesome aftermath of a dead ly gas attack in march. Part of the evidence that the syrian government has used chemical weapons against its own people. To use potential weapons of mass destruction on civilian populations crosses another line and that is going to be a game-changer. Reporter: But notice he said potential. The president said the evidence is still not definitive. We have to make these assessments deliberately. Reporter: The president has resisted taking military action in syria or even arming the rebels. That's what could change. Did the president mean it when he said use of chemical weapons would be a game-changer in syria. Reporter: What does that mean? The president made clear the use of, or the transfer to terrorist groups by the assad regime would be crossing a red line. And he retains all options to respond -- Reporter: Including military force? All options. Reporter: All options may be on the table, but the military option appears to be the last option. it clear repeatedly that the president is not going to rush into another war in the middle east over weapons of mass destruction.