Obama administration is implementing tighter security measures at foreign airports that have direct flights to the U.S. out of concern that al-Qaida is trying to develop a new and improved bomb that could go undetected through airport security.

Some questions and answers about the enhanced security measures:

Q: What's behind the move to enhance security for overseas flights bound for the U.S.?

A: The unspecified new security measures, planned a month ago, are a response to intelligence suggesting that bomb makers from al-Qaida's Yemen affiliate, known as al-Qaida in the Arabian Peninsula, or AQAP, have linked up with the al-Qaida-affiliated Jabhat al-Nusra, or the Nusra Front, in Syria to work together on bombs that can be slipped past airport security.

Q: AQAP has been trying to blow up U.S. airliners for years. What is different today?

A: U.S. officials have two new areas of concern. Thousands of Westerners, including Americans and Europeans, have traveled to fight government forces in Syria, including some who have joined up with the Nusra Front, which the U.S. considers a terrorist organization. Many of those people can board flights to the U.S. without visas. U.S. and European intelligence agencies track such people and sometimes put their names on no-fly lists, but they don't know all the names.

Second, U.S. intelligence has observed new linkages between AQAP, which possesses sophisticated bomb-making expertise, and the Nusra Front, including AQAP operatives traveling to Syria. There appear to be indications that AQAP's bomb makers are testing new designs for devices that can get past airport security, as their previous devices have done. U.S. officials say the new threat is not related to Iraq or the extremist group fighting there.

Q: What's the history of AQAP's attempts against U.S. aviation?

A: AQAP has successfully placed three nonmetallic bombs on U.S.-bound airliners, none of which detonated. Nigerian Umar Farouk Abdulmutallab smuggled an underwear bomb onto a Detroit-bound passenger jet in December 2009, but it did not detonate, and he is serving life in prison. Two other bombs were found hidden in printer cartridges on U.S.-bound cargo planes in 2010.

Q: What else is known about AQAP and bombs?

A: The underwear and printer bombs are believed to have been the work of Saudi militant Ibrahim al-Asiri, who is often described as a master bomb-maker. U.S. officials say he is continually trying to perfect his craft. The U.S. obtained a more recent design for an underwear bomb after an operation by various intelligence agencies in 2012 that placed an agent in Yemen who won AQAP's trust and smuggled out the device. Al-Asiri is believed to have trained others, so the threat is not limited to him alone.

In 2009, al-Asiri's brother, Abdullah, blew himself up in the Jeddah office of Prince Mohammed bin Nayef, then Saudi Arabia's interior minister. The bomb, believed built by Ibrahim al-Asiri, has been described as having been either hidden in a body cavity or his underwear. Nayef was unhurt.

Q: What is the U.S. doing to counter the threat?

A: The CIA and the military work closely with the Yemeni government on counterterrorism operations and run parallel drone strike campaigns in Yemen. The most recent drone strike in Yemen was reported to have occurred on June 14, killing four militants. Yemeni forces have conducted a series of ground operations in recent months, capturing and killing AQAP militants. But AQAP remains a potent force, and its operatives recently released a video in which they talked about targeting American airplanes, said a Yemeni official who would not speak publicly because he was not authorized to be quoted.

The U.S. is not known to have undertaken lethal operations against al-Qaida figures in Syria. The Obama administration has condemned both the Bashar Assad government and the extremist militants fighting it. The CIA has been running a covert program to arm and train moderate Syrian rebels, but the Nusra Front has been the most successful fighting force in the rebel movement.