Officials say jihadists are exploiting the growing upheaval as government authority disintegrates and the impoverished Arab country teeters on the brink of civil war.
With al-Qaida in the Arabian Peninsula actually seizing territory, particularly in the restive south, the Americans have stepped up their covert air war against the militants in recent days after a yearlong pause following several bungled missions.
The CIA, which carried out the first airborne assassination of an al-Qaida leader in Yemen -- and the first in the war on terror -- in 2002, is reported to be setting up a secret base in the Persian Gulf to mount air attacks on AQAP.
One of the main targets these days is U.S.-born jihadist ideologue Anwar al-Awlaki, implicated in several attacks on the United States, including the Dec. 25, 2009, attempted suicide bombing of a U.S. airliner over Detroit.
Officials say he has headed a jihadist takeover of a large swath of Abyan province in the south in recent weeks.
A U.S. unmanned aerial vehicle fired Hellfire missiles May 5 at a building in Shabwa province in which he was believed to be hiding but security sources said it failed to kill him because of a malfunctioning motor.
Awlaki, AQAP's spiritual guide, survived at least three earlier attempts to kill him from the air. Shabwa is the home of Awlaki's family and he has been sheltered by the tribes there.
U.S. President Barack Obama approved a Dec. 24, 2010, strike against a compound where Awlaki was through to be meeting regional al-Qaida leaders.
The U.S. operations in Yemen had been largely cloaked with secrecy for several years. But in six weeks in late 2010, U.S. officials say six of 15 top leaders of AQAP were killed in U.S. airstrikes.
The Americans also allegedly have an unknown number of Special Forces on the ground in Yemen, part of the U.S. military's clandestine Joint Special operations Command, whose main mission is tracking and killing terrorists.
Several months ago, U.S. officials said those troops didn't take part in operations in Yemen but helped plan missions and provide expertise, training and weapons to Yemeni commandos.
That may well have changed as the crisis in Yemen has deteriorated as AQAP operatives have taken a higher profile, even seizing cities and towns in south Yemen such as Zinjibar, capital of Abyan province on the Arabian Sea coast.
AQAP is believed to have around 400-500 hardcore operatives but can count on the support of large numbers of tribesmen who traditionally live outside government control.
The first drone attack carried out by the United States in the war against al-Qaida was made in Yemen Nov. 3, 2002. It killed a senior militant named Abu Ali al-Harithi and five other al-Qaida operatives as they drove in the desert east of Sanaa, Yemen's capital.
The mission, authorized by then President George W. Bush, was conducted by the CIA using an RQ-1 Predator UAV armed with Hellfires.
It was launched from the U.S. military base in Djibouti across the Gulf of Aden from Yemen.
Harithi was suspected of playing a key role in the suicide bombing of the USS Cole in Aden harbor Oct. 12, 2000, in which 17 sailors were killed.
Despite an outcry over the legality of that mission, it set the pattern for an aerial campaign by robot aircraft against al-Qaida and its allies that has become the primary method of killing militant chieftains.
Two weeks ago on June 3, U.S. warplanes killed another jihadist, also named Abu Ali Harithi, a mid-level AQAP leader, and several of his men in south Yemen.
U.S. operations in Yemen were considerably curtailed by President Ali Abdullah Saleh, who would have been politically compromised had he been seen to give U.S. forces free rein.
In the past, he had often used Islamist radicals to support him against his enemies, including secessionists and socialists in the south.
But with Saleh, 69, now out of action after being severely wounded June 3 in an explosion in the compound of his presidential palace in Sanaa, the Americans may have a freer hand against the militants.