Al-Qaeda in the Arabian Peninsula, at the heart of a security alert that has shut US missions, is seen by Washington as the most active branch of the jihadist network.
It was formed in January 2009 as a merger of the Yemeni and Saudi branches of Al-Qaeda and is led by Nasser al-Wuhayshi.
Based in Yemen, AQAP has a track record of launching attacks far from its base, including a bid to blow up a US airliner over Michigan on Christmas Day in 2009.
Five months earlier, an AQAP suicide bomber tried to assassinate Saudi Prince Mohammed bin Nayef, the oil-rich kingdom's current interior minister who had led a crackdown on the militant group between 2003 and 2006.
The attacker managed to infiltrate Prince Mohammed's security in Jeddah and detonate explosives planted inside his body. The prince escaped with light wounds, while the bomber was the only fatality.
In November 2010, the group claimed responsibility for sending parcel bombs to the United States and putting a bomb aboard a UPS cargo plane that crashed two months earlier in the Gulf emirate of Dubai.
The United States shut 25 of its embassies and consular offices across the Middle East and Africa on Sunday, after US intelligence uncovered what lawmakers said was the most serious threat of an Al-Qaeda attack in years.
The closure of the missions has since been extended, with 19 diplomatic outposts to remain closed until Saturday "out of an abundance of caution", the State Department said.
Among the missions to remain closed this week is the US embassy in Yemen, the ancestral home of slain Al-Qaeda founder Osama bin Laden.
The State Department said attacks were possible "particularly in the Middle East and North Africa, and possibly occurring in or emanating from the Arabian Peninsula."
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AQAP took advantage of the weakness of Yemen's central government during an uprising in 2011 against now-ousted president Ali Abdullah Saleh to seize large swathes of territory across the south.
But after a month-long offensive launched in May last year by Yemeni troops, most militants fled to the more lawless desert regions of the east towards Hadramawt province.
Yemeni-born American radical Islamic cleric Anwar al-Awlaqi, a key figure in AQAP was killed in a drone strike in Yemen in September 2011.
A few months later, Yemeni Al-Qaeda leader Fahd al-Quso, who was believed to have helped mount a deadly attack on a US warship in a Yemeni port in 2000, was killed in an air raid blamed on the US.
In mid-July AQAP confirmed the death in a US drone strike of its deputy leader Saeed al-Shehri, a former Guantanamo Bay prisoner in Cuba who had undergone rehabilitation in Saudi Arabia after his release.
The United States, the only country to operate drones in the region, has sharply increased its use of them against Al-Qaeda targets in Yemen over the past two years.
The latest one came on August 1, as Yemeni President Abdrabuh Mansur Hadi prepared to hold talks in Washington with US counterpart Barack Obama, killing four Al-Qaeda suspects. It was the third such strike in five days.
US drone strikes in Yemen nearly tripled in 2012 compared to 2011, from 18 to 53, according to the New America Foundation, a Washington-based think-tank.
The first known attack of Al-Qaeda in Yemen dates back to 1992, when bombers hit a hotel that formerly housed US Marines in the southern city of Aden, in which two non-American citizens were killed.
In 2000, an Al-Qaeda suicide attack on the naval destroyer USS Cole in the port of Aden killed 17 US military personnel.
Two years later, a bomb-laden boat struck the French-owned oil tanker Limburg off the coast of Yemen. Al-Qaeda claimed responsibility for the attack that killed a Bulgarian sailor.
Wuhayshi in July 2011 reaffirmed the group's allegiance to Ayman al-Zawahiri, head of the worldwide Al-Qaeda network since Bin Laden's killing in May 2011.