Al Qaeda and other extremist groups have increased their presence in Syria in recent months to aid rebels trying to overthrow that country's government and solidify their standing in the region, senior analysts and U.S. officials said.

The organizations' ultimate goal is to create an Islamist state once Syrian President Bashar Assad is deposed, said Ed Husain, senior fellow for Middle Eastern studies at the Council on Foreign Relations.

Meanwhile, rebel commanders fighting to oust Assad said Thursday they moved out of the strategic district of Salah al-Din as fighting intensified in the northern city of Aleppo. Syrian military aircraft continued to pound rebel strongholds, though the anti-government forces continue to hold other parts of the city.

Al Qaeda forces are flowing into Syria from Iraq, Jordan, Saudi Arabia and from as far away as England, Husain said. Al Qaeda leader Ayman al-Zawahiri recently called on fighters to join with the Free Syrian Army in its effort to overthrow Assad, potentially raising the stakes for the U.S., which hasn't done much beyond encouraging the rebels, a U.S. official said on the condition that he not be named.

"We failed early on to have better insight into who opposition members were in Syria prior to the rebellion," the official said. "Now we need to move quickly to see who we can trust before the waters get muddier."

Since the uprising began in March 2011, U.S. officials have been debating what type of support to give the rebels. Some senior congressional officials, who criticize the Obama administration for not doing enough to help the rebels, would like to see the U.S. provide anti-government forces with anti-aircraft weapons that could be used against Assad's military.

It doesn't necessarily mean that Islamists will succeed, but "we're at risk here of ignoring the rise of a jihadist regime ... and, with or without Assad, al Qaeda will play an important role for months to come," Husein added.

Secretary of State Hillary Clinton warned this week that any outside involvement in Syria must avoid escalating the sectarian violence there. "Those who are attempting to exploit the misery of the Syrian people, either by sending in proxies or sending in terrorist fighters, must recognize that that will not be tolerated first and foremost by the Syrian people," Clinton said.

Sectarian divides in Syria have intensified over the past six months and now threaten to spill over into neighboring countries, potentially further destabilizing the region, officials warned.

Sara A. Carter is The Washington Examiner's national security correspondent. She can be reached at