WASHINGTON – President Obama is under increased pressure to intervene in Syria amid allegations that President Bashar Assad’s government used chemical arms in an attack opposition groups say killed 1,300 people.

Obama said on CNN he would “think through strategically” on how to respond to the “very troublesome” allegations. British Foreign Secretary William Hague put his nation’s weight behind the reports, with a public accusation that Assad’s government used toxic gas in its attack this week on the Damascus suburb of Ghouta.

Additional Photos

Columns of smoke rise from heavy shelling in the Jobar neighborhood in west Damascus, Syria, on Thursday. President Bashar Assad’s forces pressed on with a military offensive in eastern Damascus on Thursday, bombing rebel-held suburbs. The Associated Press

Obama has maintained a cautious approach since saying a year ago that use of chemical weapons by the Assad regime would cross a U.S. “red line.” The latest violence may force him to make a decision on escalating U.S. involvement.

“This is a fork in the road for the U.S.,” said former undersecretary of state Nicholas Burns, now a professor at Harvard’s Kennedy School. “Our basic credibility is at stake. If we do nothing, the world will lose confidence in American leadership.”

Internet video and photos showed dead Syrians without visible wounds following the Aug. 21 attack, and reports from local doctors were consistent with nerve gas or lethal exposure to pesticides.

“We do believe that this is a chemical attack by the Assad regime on a large scale,” Hague said in televised comments Friday in London.

French Foreign Minister Laurent Fabius Thursday urged the world to respond “with force” to any use of chemical weapons.

After the U.S. intelligence community concluded in June that the Syrian government had used chemical arms on a “small scale” last year, killing at least 100 to 150 people, Obama said he would step up U.S. assistance for opposition forces.

Obama campaigned for office as a critic of the Iraq war and as president has set a timeline for withdrawing U.S. forces from Afghanistan. He has been reluctant to risk another war in the Middle East and has been wary that chaos in Syria following a collapse of Assad’s regime would provide a spawning ground for extremist groups.

In an interview broadcast Friday on CNN’s “New Day” program, Obama said the new reports of chemical weapons use get to “core national interests that the United States has, both in terms of us making sure that weapons of mass destruction are not proliferating, as well as needing to protect our allies, our bases in the region,” he said.

Obama stopped short of saying what his response would be.

“We have to think through strategically what’s going to be in our long-term national interests, even as we work cooperatively internationally to do everything we can to put pressure on those who would kill innocent civilians,” he said.

Bernadette Meehan, a spokeswoman for the Obama’s National Security Council, said the president “has directed the intelligence community to gather facts and evidence so that we can determine what occurred in Syria.” She added that “we have a range of options available, and we are going to act very deliberately.”

The president’s advisers are discussing options that don’t at the moment include imposing a no-fly zone on Syria or putting U.S. troops inside the country, said an administration official who asked for anonymity to talk about the closed-door deliberations.

One option administration officials are considering is more active support for Syria’s neighbors, said another U.S. official involved in Middle East policymaking.

While no decisions have been made, consideration is being given to asking Congress to approve increased military and humanitarian assistance to Jordan, Turkey and possibly Iraq, the official said.

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