"A lot of things that the pro-interventionist crowd had argued two years ago have come to pass," said Shadi Hamid, a Brookings Institution expert who called for military intervention in 2012. "The argument was that radicalism will rise."

It is impossible to know whether a Libya-like intervention would have ended the conflict in Syria or exacerbated it. But citing recent statements from administration officials, Hamid argued that the current American approach is not working.

In his testimony last week, Clapper said that American intelligence agencies had picked up indications of "training complexes" within Syria "to train people to go back to their countries and conduct terrorist acts, so this is a huge concern."

The retired Air Force general estimated that more than 7,000 foreigners from 50 countries—"many of them from Europe and the Mideast"—are fighting in Syria. He compared rebel-controlled parts of northern Syria to the Federally Administered Tribal Areas of Pakistan, or FATA, where foreign and local militants have sheltered since the fall of the Taliban in 2001. "What's going on there may be in some respects a new FATA," Clapper said. "And the attraction of these foreign fighters is very, very worrisome."

In the past, Clapper has been accused of exaggerating terrorist threats and making misleading statements about the scope of American surveillance activities. But Clapper is not the only senior official expressing concern about the rising militant presence in Syria.

At a private meeting with members of Congress at the Munich Security Conference last week, Secretary of State John Kerry said that "the al-Qaeda threat is real, it is getting out of hand," Republican U.S. Senator Lindsey Graham later told reporters. "He openly talked about supporting arming the rebels. He openly talked about forming a coalition against al-Qaeda because it's a direct threat." State Department officials said that Graham and other members of Congress who disclosed the private meeting distorted Kerry's statements. They denied that Kerry raised arming the rebels or described the current policy as a failure.

Noah Bonsey, a Beirut-based senior analyst for the International Crisis Group, called Kerry's reported statements "an acknowledgement of the facts." On the rebel side of the conflict, al-Qaeda-aligned militants have badly damaged the international reputation of the Syrian opposition. On the government side, Assad and his backers in Iran and Russia are increasingly confident.

"Geneva made abundantly clear that the regime is not prepared to compromise on anything at all, no matter how small," Bonsey said in a telephone interview, referring to the peace talks. "They believe themselves to be winning and they perceive themselves as seeing no real pressure, certainly not from Iran and probably not from Russia."