WASHINGTON (AP) -- The U.S. has pummeled terror leaders in Yemen with targeted drone strikes and relaunched efforts to send military aid to the struggling nation, but the Yemen-based al-Qaida in the Arabian Peninsula has proven to be a tenacious enemy.
Gathered in small cells scattered across Yemen's vast under-governed regions, AQAP is at the heart of what American officials say is a credible and specific terrorist threat that shut down U.S. diplomatic facilities in 19 cities across the Mideast and Africa.
Officials say the core al-Qaida leader, Ayman al-Zawahri, who took over for Osama bin Laden and works from Pakistan, has reached out to the Yemeni branch, cementing their ties and further signaling the AQAP is once again looking to target U.S. and Western interests after a sustained period of more local and regional focus.
For puzzled Americans who've been told that al-Qaida is on the decline, the latest warnings raise questions about how successful America's war on terror has been and whether the terror group has been able to reorganize and reconstitute itself since bin Laden's death in May 2011.
And, although U.S. officials agreed a year ago to restart military aid to Yemen, it's unclear how much of the new aircraft and weapons have arrived. After aid to Yemen was frozen for some time, the U.S. military is once again on the ground there training Yemeni special operations forces and has delivered more than a dozen helicopters to the Yemeni military and provided training for them, U.S. defense officials said.
But other weapons and equipment are still in the pipeline, according to a Mideast official.
The latest terror alert was triggered in part when a secret message between al-Zawahri and AQAP leader Nasser al-Wahishi was intercepted several weeks ago, according to a U.S. intelligence official and a Mideast diplomat. The officials spoke on condition of anonymity because they were not authorized to discuss the matter publicly.
There long has been movement of fighters between Pakistan and Yemen, and discussions between the two groups, but the latest communication triggered worries and prompted the U.S. to take steps to boost security. The embassy closures came one day after a meeting between President Barack Obama and Yemeni President Abdo Rabu Mansour Hadi.
AQAP has been widely considered al-Qaida's most dangerous affiliate for several years. Even though the group lost Anwar al-Awlaki -- one of its key inspirational leaders -- to a U.S. drone strike in 2011, al-Wahishi and the group's master bomb maker, Ibrahim Hassan al-Asiri, remain on the loose and determined to target the U.S. and other Western interests.
The group is linked to the botched Christmas Day 2009 bombing of an airliner bound for Detroit and explosives-laden parcels intercepted aboard cargo flights a year later -- both incidents involving al-Asiri's expertise.
"This does indicate a level of cooperation that, while perhaps not surprising, a lot of people were debating with al-Awlaki's demise that AQAP may look a little more locally and regionally," said Frank Cilluffo, director of a homeland security studies program at George Washington University. "But now it appears that they're not only thinking global, acting local, but thinking global, acting global yet again."
Cilluffo, a former security adviser to President George W. Bush, said communications between those two leaders fuels worries that "we have a significant threat on our hands."
In recent years, however, AQAP has been focused more on making gains at home, taking advantage of an unstable government and overstretched military that was forced to concentrate on protecting the political center in Sanaa. As a result, said a senior defense official, AQAP was able to expand its foothold in the south, capture more weapons and gain control of additional territory.
U.S. officials believed that the loss of Awlaki and other leaders forced the group to focus more on internal threats, even though their desire to target Western interests continued. Al-Wahishi, al-Asiri and other leaders have proven to be strong and committed fighters, the official said, making them both capable and dangerous and willing to use their time and resources to further hone their bomb-making skills. The official was not authorized to speak publicly about intelligence matters so spoke on condition of anonymity.
It's unclear what the latest communications between al-Zawahri and al-Wahishi means in terms of actual terror attacks, and whether it suggests any attempt by the core al-Qaida leader to either control or enlist aid from AQAP. But it suggests the lines could be blurring between the two groups.
Obama and others have consistently alluded to the weakening of core al-Qaida in Pakistan -- particularly since a Navy SEAL team killed bin Laden in Pakistan two years ago. Obama frequently touts bin Laden's death in his speeches, and has declared that the terror group was "on the path to defeat."
But while core al-Qaida may be on the ropes, officials have warned repeatedly that its offshoots in places like Yemen and Africa, as well as homegrown believers in the U.S., have grown increasingly dangerous and more difficult to track.
"There has been a growth in the number and geographic scope of al-Qaida affiliates and allies over the past decade, indicating that al-Qaida and its brand are far from defeated," Seth G. Jones, a terrorism expert at RAND Corp., said in written testimony July 18 to the House Foreign Affairs terrorism subcommittee. "This growth has been facilitated by the Arab uprisings, which have weakened regimes across North Africa and the Middle East."
U.S. drone strikes have been steadily targeting AQAP, picking off a variety of mainly mid-level insurgents, including three attacks in the past week that hit cars believed to be carrying al-Qaida members in Yemen's restive south. The strikes, while conducted by U.S. Predator or Reaper drones, are carried out in cooperation with the Yemenis.
On Monday, officials declined to be more specific about the latest threat.
White House spokesman Jay Carney wouldn't say whether the threat extends to the United States or whether Americans should be fearful because of the alerts.
"What we know is the threat emanates from, and may be focused on, occurring in the Arabian Peninsula," Carney said. "It could potentially be beyond that, or elsewhere."
The State Department on Sunday closed a total of 19 diplomatic posts until next Saturday. They include posts in Bangladesh and across North Africa and the Middle East as well as East Africa, including Madagascar, Burundi, Rwanda and Mauritius.
Associated Press writers Lara Jakes, Kimberly Dozier, Robert Burns and Julie Pace contributed to this report.