By Barbara Starr

The Pentagon has in recent days stepped up planning for potential military intervention in the Syrian civil war, specifically because of growing evidence the regime may have used chemical weapons, CNN has learned.

"There is intensified planning in the works as more precise information comes in on the Syrian regime's potential use of chemical weapons and the body of evidence grows," a senior administration official said.

The official, who has direct knowledge of the effort, declined to be identified because of the sensitivity of the situation.

If President Barack Obama were to order action, it could involve thousands of U.S. troops. But all of the options face serious military challenges.



The official said it's not likely to involve troops on the ground in Syria. Two other officials say the most likely options would be using cruise missiles based at sea and fighter jets to try to destroy chemical sites or the headquarters of Syrian military elements linked to them.

But even that type of option has grown increasingly complex in recent weeks as the Syrian regime has moved chemical stockpiles around the country.

"We have to now assume they are going to continue to do so," the official said, noting that Joint Chiefs Chairman Gen. Martin Dempsey has already told Congress the military could not secure the entire chemical stockpile because it does not know where it's all located.

Defense Secretary Chuck Hagel and other senior civilian officials at the Pentagon have sought updated military options in the last few weeks as suspicions and intelligence grew that the Syrian regime had used sarin gas on its own people.

The official said Obama so far has not asked for those options, but the Pentagon was moving ahead to be prepared.

"As the situation in Syria becomes more grave and as we are increasingly concerned about chemical weapons use in Syria, it's the responsibility of the U.S. military to prepare detailed options," the official said.

Hagel told reporters on Monday that he would not speculate any options regarding Syria.

The updated planning is separate from recent discussions with Congress several weeks ago about options for Syria, the official said.

There are also specific new discussions with Israel, Turkey and Jordan about what the official called the "realities of the conflict."

These discussions center around what to do if chemical weapons become a risk to their populations and also what immediate actions would have to be taken if the regime of Syrian President Bashar al-Assad were to suddenly fall.

The new planning, he said, is an effort to "align" military capabilities with various options and scenarios that could likely emerge.

But he also emphasized it also involves looking at more direct U.S. military involvement in providing humanitarian relief for the hundreds of thousands of Syrian refugees now in Jordan and Turkey.

A previously announced deployment of some personnel from the First Armored Division headquarters to Jordan is part of this stepped up planning effort.