Transcript for President Trump and the US get a warning from Russia and Iran over Syria

We turn to an image Syria is quite proud of this evening, planes taking off from very airfield targeted by U.S. Cruise missiles last Thursday night. Those jets taking off from that same base. And there was a moment today at the white house when our correspondent asked is there a new red line on Syria? The white house answered but then changed the answer later this afternoon. Here's ABC's senior white house correspondent Cecilia Vega. Reporter: President trump picking up the phone to congratulate the commanding officers aboard those Navy ships that launched 59 missiles into a Syrian airfield. But from the looks of these images on Russian state TV, that airbase is back in business, planes taking off, once again. Tonight, new accusations that Russia was well aware of bashar Al Assad's chemical weapons stockpile. I believe that Russia knew about the chemical weapons because they were operating from exactly the same base. Reporter: This as the white house seems to shift its red line on Syria, saying it's not just the use of chemical weapons that would provoke an American response. Is the red line -- just to clarify, the red line for this white house chemical warfare? Is conventional warfare enough to get the president to go further there than -- than this white house has gone before? The answer is, is that if you gas a baby, if you put a barrel bomb in to innocent people, I think you can -- you will -- you will see a response from this president. That is unacceptable. Reporter: But Assad has frequently used barrel bombs against his own people, nearly 13,000 last year alone, according to human rights groups. Spicer's statement, not the only mixed message on Syria from the trump administration, top officials, divided on whether or not Assad should stay or go. Secretary of state Rex tillerson saying it's up to the Syrian people. We believe the Syrian people will lawfully be able to decide the fate of bashar Al Assad. Reporter: But U.N. Ambassador Nikki Haley all but advocating for a regime change. It's going to be hard to see a government that's peaceful and stable with Assad. Reporter: The white house now seeming to echo that. How far is the president willing to go to see bashar Al Assad out of power there? So, just to be clear, I can't -- I don't think it's -- it's -- you can't imagine a -- a stable and peaceful Syria with Assad as -- as -- in charge. Reporter: Tonight, from Syria, Russia and Iran, a joint statement, accusing the U.S. Of crossing a red line with its missile strikes, warning, "From now on, we will respond to anyone, including America, if it attacks Syria and crosses the red lines." It comes as secretary of state tillerson heads to Moscow for talks. As Exxon CEO, tillerson once shared champagne with Vladimir Putin. But this time, there's no plans for the two to meet face-to-face. Cecilia live with us from the white house. We heard Sean spicer tell you if Al Assad barrel bombs innocent people that president trump will respond. But tonight, the white house walking that back now? They certainly are, David. In fact, they're emphasizing that quote, this is not a shift in U.S. Policy. They say nothing has changed if in fact the Assad -- the white house decided to go after Assad for the use of barrel bombs. This would mark an incredible shift in U.S. Policy. David. All right, Cecilia Vega with us from the white house tonight.

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