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Obama: There is 'some evidence' Syria's Assad used chemical weapons

President Barack Obama expands on what his administration is doing in response to reports that chemical weapons may have been used by the Syrian regime.

President Barack Obama said Tuesday that the United States has evidence that chemical weapons were used in Syria’s brutal civil war but that it remains unclear who used them.

In a White House press conference, Obama said there is “some evidence” that the regime of Syrian President Bashar Assad used the weapons, but urged against rushing to judgment, saying more facts must be known before any action taken.

“We don't know how they were used, when they were used, who used them," the president said, adding "we don't have a chain of custody that establishes what exactly happened."

He reiterated that the administration needs more intelligence before before he’s willing to entertain any kind of escalation in Syria.

“When I am making decisions about America’s national security and the potential for taking additional action in terms of chemical weapons use, I’ve got to make sure I’ve got the facts,” he said.

As in the past, Obama referred to chemical weapons use as a “game-changer.” He gave no indication, however, that the United States or its allies would step up action against the Assad regime, adding that as long ago as last year he had asked the Pentagon to prepare options on Syria. Obama declined to elaborate on any plans in the works.

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A look back at the conflict that has overtaken the country.

“By ‘game-changer’ I mean that we would have to rethink the range of options that are available to us,” he said.

“Obviously, there are options … that are on the shelf right now that we have not deployed,” the president added.

Obama's caution about launching any military action against Syria in the absence of firm evidence of the use of chemical weapons likely stems from his predecessor, President George W. Bush, who has been criticized for starting the Iraq war over claims of weapons of mass destruction that never materialized.

"If we end up rushing to judgment without hard, effective evidence, then we can find ourselves in the position where we can't mobilize the international community to support what we do," he said.

The primary incident of alleged chemical weapons use came on March 19, when rebel and government forces were engaged in heavy fighting in a strategically important town near the city of Aleppo.

Syria’s state news agency SANA said 25 people were killed and dozens more were injured; they blamed the attack on rebels.

Rebel forces, meanwhile, said government forces had delivered a chemical agent with a Scud missile.

Both sides described civilians panicking, having breathing difficulties and convulsing, and both sides continue to blame each other.

Regional reaction
Countries in the region have been watching with growing concern at the worsening conflict in Syria, none more so than Israel. 

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People resisting the army of President Bashar al-Assad in northern Syria cope with loss and prepare for fighting.

"The main risk for Israel is that chemical weapons end up in the hands of the Hezbollah or the hands of any jihadist," said Dr. David Friedman, a chemical and biological weapons expert at the Institute for National Security Studies in Tel Aviv. "If they do then all the world needs to be afraid of this."

Nonetheless, he said, Israel wasn't pressuring Obama to intervene in Syria.

Gamal Abdel Gawad, a political science professor at the American University in Cairo, said that Obama would likely not react with an all-out military intervention.  

"Its a good idea (not to intervene militarily). The risks are very high," he said.  "Rule number one in military intervention is to know how to get out. There is no answer to this question. You are playing with adversaries that have a lot of potential and resources."

Obama’s comments came on a day when a bombing in central Damascus killed at least 13 people and injured scores more, according to state media and the Britain-based Human Rights Watch.

Syria’s two-year civil war has claimed more than 70,000 lives, according to the United Nations.

The U.S. and other countries have provided aid to rebels but have stopped short of committing to further involvement in the battle to overthrow Assad.

NBC News's Paul Goldman and Charlene Gubash contributed to this report. 

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Obama cautions against rush to action in Syria

Obama reiterates chemical weapons would be 'game-changer'

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