By Barbara Starr

Intelligence increasingly shows al Qaeda is encouraging American and other western fighters in Syria to undertake special training to prepare them for possibly returning to their home countries to carry out attacks, U.S. officials told CNN.

National security officials are concerned that amid the mayhem of Syria's civil war, the global terror group has found a safe haven and a place where it again can recruit, plot and train operatives to carry out attacks on the U.S. homeland.

"There is an effort to exploit them to get them to return to the West," one official said.

The official, who did not speak for full attribution, described scenarios in which foreign fighters arrive in Syria and are recruited by al Qaeda to train for external attacks.

CNN has spoken to a number of officials who sketched out similar scenarios.

It is not certain how many Americans may have been singled out or if any have been successfully recruited. But another worry is that any Europeans in the mix may be a potential threat as well because they don't need visas to enter the United States.

Training facilities can change locations and related materials can be found online, minimizing chances that a specific target could be identified and attacked.

Yemeni affiliate in Syria

Another concern is the presence in Syria of fighters and operatives affiliated with Al Qaeda in the Arabian Peninsula (AQAP), which is based in Yemen.

Its top bomb maker, Ibrahim al-Asiri, is believed to have developed underwear bombs and an explosive hidden in printer cartridges.

Just this week, U.S. authorities warned airlines about possible shoe bombs. One law enforcement official said recent intelligence points to tactics believed to be tied to al-Asiri.

There is concern that new bomb-making skills could be passed along to those already receiving training in suicide bomb attacks.

Even if attacks against the American homeland are not imminent, officials say new training in Syria could present a longer-term concern if potential terrorists return with new training and wait to make bombs or mount an attack.

The United States estimates up to 70 Americans have traveled to Syria, many with the purpose to join extremist groups.

Focusing on foreign fighters

U.S. homeland security and intelligence officials are trying to track them entering Syria and hope to do so, if they return to the United States.

Department of Homeland Security Secretary Jeh Johnson earlier this month called Syria a "homeland" security threat, reflecting a more direct concern by Obama administration officials who previously described the war-torn country as a regional threat that impacts U.S. national security interests abroad.

"We are focused on foreign fighters heading to Syria right now. Based on our work and the work of our international partners, we know individuals from the U.S., Canada and Europe are traveling to Syria to fight in the conflict," Johnson said.

"At the same time, extremists are actively trying to recruit westerners, indoctrinate them, and see them return to their home countries with an extremist agenda mission," he added.

Johnson made those remarks within hours of returning from Poland where European Union ministers met specifically to discuss foreign fighters traveling to and from Syria.

Director of National Intelligence James Clapper raised the issue as well in recent testimony before the Senate Intelligence Committee.

"We're seeing now the appearance of training complexes in Syria to train people to go back to their countries, and, of course, conduct more terrorist attacks," he said.

But officials told CNN the public comments only begin to address concerns growing within U.S. and European intelligence agencies.