Nightly News   |  April 28, 2013

Pressure mounts for action in Syria

Republican lawmakers on Sunday continued their push for U.S. intervention in Syria. NBC’s Peter Alexander reports.

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>>> still slowly extricating itself from the war in afghanistan and looming threats in north korea and iran, some hard questions being asked tonight about what, if anything, the u.s. can and should do militarily to end the conflict in syria . the subject has reemerged on to the front burner following indications the syrian regime may have used chemical weapons against its own people. last summer the obama administration suggested such a step would be a trigger for intervention. now the white house wants stronger evidence, even as the call grows louder in washington to act now. nbc's peter alexanderer has more from the white house .

>> reporter: inside syria 's civil war , another round of violent clashes today. at least seven rebels reported dead. just days after the u.s. said evidence suggests ba sar al asad 's regime has used chemical weapons . president obama is facing growing pressure to act. with republican lawmakers pushing several options.

>> if you could neutralize the aerial advantage, i think you could turn the tide of battle.

>> a safe zone. arming the rebels. making sure we help with the refugees. i do want to give them the assistance which would give them a dramatic shift in the balance of power in syria .

>> reporter: president obama has warned syria that using chemical weapons would be a game changer.

>> a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized. that would change my calculus.

>> the president's laid down a line. more than just syria , iran is paying attention to this, north korea is paying attention to this.

>> reporter: in today's " washington post ," anne marie slaughter, a former obama state department official insists the game has already changed and that u.s. credibility is on the line. obama must realize the tremendous damage he will do to the u.s. and to his legacy if he fails to act," she writes. but white house officials say they still have too many questions about the origin of the weapons and how they were used. this weekend syrian leaders dismissed u.s. claims that they've used chemical weapons , calling that a bare-faced lie and adding to the complexity they blame the rebels for chemical attacks. as they weigh their options, administration officials stress the need for caution. a decade after faulty intelligence contributed to the u.s. decision to iraq iraq. nicholas burns is a former bush administration official and diplomat.

>> we do need to learn the lessons of history before we use military force . we have to be absolutely sure of the reasons for which we use it. are.

>> reporter: about those calls for stronger action, lester -- republican senators said they are against sending american troops into syria . in the words of john mccain today -- that is the thing the u.s. could do right now.

>> peter alexander , thank you.