Transcript for U.S. weighs military action in Syria following chemical weapons attack

As you heard Martha if any action taken comes with great risk. Obama weighed military action before. Now Russians are involved and military troops. What are the greatest risk? Here's the ABC's Brian Ross. Reporter: With a prime target the specific military unit that guarded and deployed weapons. Unit 450, of the Syrian air force. If you are going to strike in a way that is directly related to chemical weapons use, you'd have to hit some part of unit 450. Reporter: The horrific images this week from Syria put a lie to the Syrian and usa claims that the country's stockpile of sarin gas and other chemical weapons has been destroyed. But even with a diminish supply, a direct strike on whatever chemical weapons remain still runs a serious risk. If you are going to hit them, if you are going to blow them up, you know, are you going to create agent plumes, which are going to damage, you know, civilian areas. Reporter: The U.S. Attack plan in 2013 included Syrian airfields, tank depots and even the ministry of defense in downtown Damascus. They can go right through a ten-foot block of that building, so right into a particular office. Reporter: But any attack now is complicated by the presence of the Russian soldiers and advisers, across Syrian military installations. And adding to the complexity, is zblet's bring in Brian Ross. We don't know if any action is going to be taken tonight. But we did hear those words just 24 hours ago from the U.N. Ambassador, Nikki Haley asking how many children have to die before they care? Is that a test of that relationship too. In fact the world is watching to see if the president is now prepared to challenge Russia and its leader who he has praised so much, Vladimir Putin.

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