President Obama made it crystal clear last month that he would not tolerate Syrian use of chemical weapons, telling reporters, "I have made clear the use of chemical weapons is a game changer ."

But what does that mean?

It's not an academic question. There are now three U.S. allies - Britain, France and Israel - who say Syria has used chemical weapons. The British and French went so far as to tell the United Nations last week that they have evidence in the form of soil samples that Syria used chemical weapons in attacks on Aleppo and Damascus on March 19. They have also accused the Syrians using chemical weapons in the city of Homs on Dec 23.

And today Israel's top intelligence official said he believes Syria has used chemical weapons multiple times. Based on observations of the victims - including shrunken pupils and foaming at the mouth - he believes the Syrians have used the lethal nerve agent sarin as well as other less lethal chemicals.

Does the White House agree with its allies? If so, what is President Obama going to do about it?

I asked White House Press Secretary those questions today the daily press briefing. As you can see, he's not prepared to say what exactly the President meant by saying chemical weapons use would be a game changer. Watch the exchange here .

KARL: You now have three separate American allies who say that they have evidence that Syria used chemical weapons in multiple instances going back to December and as recently as March. When the president talked about this, he didn't just say it was unacceptable, he said it was a gamechanger. So my question to you is, what does that mean? What does it mean, "gamechanger?"

MR. CARNEY: It means that we are assessing the reports of chemical weapons use, and it is very important to do whatever we can to monitor, investigate and verify any credible allegations, given the enormous consequences for the Syrian people and given, as you said, the clear - the president's clear statement about the fact that chemical weapons use is unacceptable.

It is precisely because of the seriousness of the use of chemical weapons and the seriousness with which the president made clear that that use would be unacceptable, that it is incumbent upon us and our partners to investigate thoroughly and validate or verify allegations of chemical weapons use. And we are obviously doing that.

KARL: But again, the president didn't just say unacceptable. He said chemical weapons use would be a gamechanger. So my question is, what does that mean?

MR. CARNEY: Well, again, I'm not going to speculate about actions that may or may not -

KARL: It isn't speculation.

MR. CARNEY: Sure it is.

KARL: It's something the president said. The president said it would be a game changer -

MR. CARNEY: He means that it's a red line, and the use of chemical weapons is unacceptable, and it would not be acceptable to the president, to the United States, we - all the more reason why we have to monitor very closely and take action to verify and validate credible claims of chemical weapons use. What I won't do is jump to the next step and say, if claims are verified, what action will we take? That's speculating, and I won't do that. But you can be sure, based on what the president told you from this podium, that this is a very serious matter, which is why we are investigating it the way that we are.

KARL: Can we be sure he's going to take action? I mean, you might tell me what the action is. Is he going to do something about this?

MR. CARNEY: Well, you're - again, you're saying do something about this. we have to make sure that we monitor - but again -

KARL : This isn't crazy speculation -

MR. CARNEY: I'm not saying it is.

KARL: You have two allies who say they have physical evidence. You have the Israelis who have made a strong case about - (inaudible).

MR. CARNEY: And we are, as we absolutely must, working with our partners to investigate these allegations. It is absolutely the case, in an environment that - like the one you have in Syria, that proving chemical weapons use can be difficult. But we are engaged in a process of trying to investigate and verify these allegations.

KARL: And what are we doing in that area? We know that the Brits and - actually went and took soil samples. What are we doing to -

MR. CARNEY: Well, I'm not going to get into the methods that we use to gather information or intelligence, but you can be sure that we are utilizing the tools that we have available to us to investigate these very serious allegations.