Is America planning to attack Syria?

The administration concedes Assad's hold on power has strengthened.

On Tuesday, Secretary of State John Kerry offered lingering hope that the peace talks could yield results.

However slowly, the potential for a terrorist base developing in northern Syria akin to Afghanistan before the Sept. 11, 2001, attacks may be changing the administration's thinking.

In recent weeks, the president's senior national security aides have delivered dire warnings about extremist havens in Syria and about Americans and other Westerners joining the fight and being radicalized.

Testifying before Congress this month, National Intelligence Director James Clapper estimated there were about 26,000 extremists in Syria, including around 7,000 foreigners, in an insurgency encompassing 75,000 to 110,000 fighters.

Jabhat al-Nusra, for example, one of the most powerful rebel factions, has “aspirations” for attacks on the United States, he said.

In addition to worries about foreigners, officials cite concerns about small numbers of Americans who've fought in Syria and returned home.