– Thousands of foreign fighters have bolstered the ranks of militant groups in Syria and Iraq in recent months, according to U.S. officials, driving fresh concern about potential terrorist plots aimed at the United States or its allies.

The surge, as well as intelligence that Yemeni terrorists have developed a powerful cellphone bomb designed to avoid detection at airports, is behind the Obama administration's increasingly urgent warnings that a European or U.S. passport holder might try to take down a passenger jet.

As many as 10,000 foreign fighters — a third more than in February — have joined the militias seeking to overthrow Syrian President Bashar Assad and the Islamic State in Iraq fighters who have swept across northern Iraq, said a counterterrorism official who spoke on condition of anonymity.

As many as 3,000 of them hold Western passports and thus can travel easily across most borders. Several dozen, perhaps as many as 100, hold U.S. passports, and officials say that number is growing.

The potential partnership between some of those Westerners and sophisticated bomb makers from the Yemen-based Al-Qaida in the Arabian Peninsula, long viewed as the terrorist network's most dangerous offshoot, has raised alarm in Washington and other capitals.

"It's something that gives us really extreme, extreme concern," Attorney General Eric Holder told ABC News. "In some ways, it's more frightening than anything I think I've seen as attorney general."

FBI Director James Comey said that the threat "keeps me up at night." He sees the region as a "launching ground" for potential Sept. 11-style attacks.

Two U.S. citizens have been arrested on terrorism-related charges since April before boarding flights to Turkey, suspected of being on their way to help the militants. One is said to have paid his way with his federal tax refund. In May, a Florida man became a suicide bomber in northern Syria.

"It's the largest number of Western fighters we have ever seen in a jihadist theater," said Seth Jones, a former U.S. counterterrorism official now with the Rand Corp. think tank.

"The scale of this is huge," said Daniel Byman, professor of security studies at Georgetown University.

Some of the Western militants are joining Al Nusra Front, an Al-Qaida affiliate that is focused on defeating the Assad government. Others have rallied to ISIL, which broke away from Al-Qaida and has seized territory in eastern Syria and northern Iraq.

Although the rebels in Syria are struggling, the stunning battlefield successes of the Islamic State fighters in Iraq, and its calls for an Islamic caliphate, have energized recruiting efforts for all the militant groups.