Story highlights Defense secretary calls for speedy probe, says if al-Assad used chemical weapons, he may do so again

Military seeks to give President Obama a range of options, depending on his response to crisis

Target lists for possible air strikes have been updated as have plans for possible cruise missiles

U.N. attempting to investigate claims of fresh chemical weapons use by Syrian regime

The U.S. military has updated options for a forceful intervention in Syria to give President Barack Obama a range of choices should he decide to deepen American involvement in a civil war where new claims surfaced this week about possible chemical weapons use by the regime.

A senior Defense Department official told CNN on Friday that target lists for possible air strikes have been updated. The planning also included updates on the potential use of cruise missiles, which would not require fighter pilots to enter Syrian airspace.

But the official cautioned the steps were taken "to give the president a current and comprehensive range of choices" and that no decisions were made at a national security meeting on Thursday at the White House.

The official said there are certain static targets, like government buildings and military installations, but that forces and equipment of Syrian President Bashar al-Assad "continue to move" and thus require flexibility in planning.

A U.N. team is in Syria attempting to investigate the latest claim of chemical weapons use outside Damascus that killed more than 1,300 people, according to the opposition.

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Obama told CNN in an interview broadcast on Friday the United States is working with the United Nations to gather information on the alleged attack, but noted that preliminary signs point to a "big event of grave concern."

"It is very troublesome," he said. "That starts getting to some 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."

Obama said there was an abbreviated time frame for making decisions about Syria.

Defense Secretary Chuck Hagel said Friday "a very quick assessment" of the allegations is important because if the Assad government did use chemical weapons, "there may be another attack coming," Reuters reported from aboard the military aircraft carrying Hagel to Malaysia.

Hagel told reporters on the plane that the military was responsible for "positioning our assets to be able to carry out different options," but he said that did not necessarily mean forces or assets had been moved, according to Reuters.

Speaking with "New Day" anchor Chris Cuomo, Obama defended his administration's decision to not intervene militarily in the conflict so far, but predicted that American focus on the country's strife would be necessary for the fighting to come to an end.

"I think it is fair to say that, as difficult as the problem is, this is something that is going to require America's attention and hopefully the entire international community's attention," Obama said.

If the United States affirmed evidence that indicated Syrian government responsibility, the U.S. military options would include targeting al-Assad's capability to deliver chemical weapons, the official added.

A White House spokesman said on Friday that the administration has long maintained that "all options remain on the table" regarding Syria. But Obama has previously indicated no plans to place American "boots on the ground."

Military planners last conducted a major update of options for Syria in April, in response to bipartisan pressure from members of Congress.

"The plans are constantly being reviewed and tweaked," the official said, adding that the latest update represented a more comprehensive review of airstrike target lists.

Sen. John McCain, an advocate for a more forceful U.S. response to the Syrian conflict, has suggested that American air power could take out runways and planes used by al-Assad's forces that he said are "dominating the battlefields and the towns and the cities."

He also said the administration could "supply the right kind of weapons to rebels to establish a 'no fly' zone" and utilize Patriot missile batteries elsewhere in the region.

So far, Obama has authorized a limited amount of military hardware for the rebels in addition to logistical and humanitarian assistance.

A chief problem has been identifying those rebels the United States would happily deal with vs elements said to be militants, including some with ties to al Qaeda.

"Syria today is not about choosing between two sides but rather about choosing one among many sides," Joint Chiefs Chairman Gen. Martin Dempsey wrote this week to a member of Congress.

"It is my belief that the side we choose must be ready to promote their interests and ours when the balance shifts in their favor. Today, they are not," he said in the August 19 letter.

Dempsey also warned that even limited military action in Syria could lead to deeper involvement.

Late on Friday, a defense official said the United States had added a Navy destroyer to the eastern Mediterranean fleet.

The USS Ramage arrived to replace the USS Mahan, but the Mahan is going to stay around a bit longer, so temporarily there will be four. The USS Gravelly and USS Barry remain.

These ships are all equipped with the Tomahawk Land Attack Missiles, a long-range subsonic cruise missile used to attack land targets.