WASHINGTON - In a devastating double blow to al Qaeda's most dangerous franchise, U.S. counterterrorism forces killed two American citizens who played major roles in inspiring attacks against the United States, U.S. and Yemeni officials said Friday.

U.S-born cleric Anwar al-Awlaki and Samir Khan, who edited a slick Jihadi internet magazine, were killed in an air strike on their convoy in Yemen by a joint CIA-U.S. military operation, according to counterterrorism officials. Al-Awlaki was targeted in the killing; Khan apparently was not targeted directly.

After three weeks of tracking the targets, U.S. armed drones and fighter jets shadowed the al Qaeda convoy, before armed drones launched the lethal strike early Friday. The strike killed four operatives in all, officials said. All officials spoke on condition of anonymity to discuss matters of intelligence.

Al-Awlaki played a "significant operational role" in plotting and inspiring attacks on the United States, U.S. officials said Friday, as they disclosed detailed intelligence to justify the killing of a U.S. citizen. Khan, who was from North Carolina, wasn't considered operational but had published seven issues of Inspire Magazine offering advice on how to make bombs and the use of weapons. The magazine was widely read.

Following the strike, a U.S. official outlined new details of al-Awlaki's involvement in anti-U.S. operation, including the attempted bombing of a U.S.-bound aircraft on Dec. 25, 2009. The official said al-Awlaki also specifically directed the men accused of trying to bomb a U.S.-bound plane to detonate an explosive device over U.S. airspace to maximize casualties. Neither attempt succeeded

U.S. officials also believe al-Awlaki had sought to use poisons, including cyanide and ricin, to attack Westerners.

The U.S. and counterterrorism officials all spoke on condition of anonymity to discuss intelligence matters.

Al-Awlaki was killed by the same U.S. military unit that got Osama bin Laden. A U.S. official said four individuals were killed in the attack.

Al-Awlaki is the most prominent al Qaeda figure to be killed since bin Laden's death in May.

U.S. word of al-Awlaki's death came after the government of Yemen reported that he had been killed Friday about five miles from the town of Khashef, some 87 miles (140 kilometres) from the capital Sanaa.

The airstrike was carried out more openly than the covert operation that sent Navy SEALs into bin Laden's Pakistani compound, U.S. officials said.

Counterterror co-operation between the United States and Yemen has improved in recent weeks, allowing better intelligence-gathering on al-Awlaki's movements, U.S. officials said. The ability to track him wbetter as a primary factor in the success of the strike, U.S. officials said. Officials spoke on condition of anonymity to discuss intelligence matters.

Al-Awlaki's death is the latest in a run of high-profile kills for Washington under President Barack Obama. But the killing raises questions that the death of other al Qaeda leaders, including bin Laden, did not.

Al-Awlaki was a U.S. citizen, born in New Mexico to Yemeni parents, who had not been charged with any crime. Civil liberties groups have questioned the government's authority to kill an American without trial.

U.S. officials have said they believe al-Awlaki inspired the actions of Army psychiatrist Maj. Nidal Hasan, who is charged with 13 counts of premeditated murder and 32 counts of attempted premeditated murder in the attack at Fort Hood, Texas.

In New York, the Pakistani-American man who pleaded guilty to the May 2010 Times Square car bombing attempt said he was "inspired" by al-Awlaki after making contact over the Internet.

Al-Awlaki also is believed to have had a hand in mail bombs addressed to Chicago-area synagogues, packages intercepted in Dubai and Europe in October 2010.

Al-Awlaki's death "will especially impact the group's ability to recruit, inspire and raise funds as al-Awlaki's influence and ability to connect to a broad demographic of potential supporters was unprecedented," said terrorist analyst Ben Venzke of the private intelligence monitoring firm, the IntelCenter.

But Venzke said the terror group al-Qaida in the Arabian Peninsula will remain the most dangerous regional arm "both in its region and for the direct threat it poses to the U.S. following three recent failed attacks," with its leader Nasir al-Wahayshi still at large.

Al-Awlaki wrote an article in the latest issue of the terror group's magazine justifying attacking civilians in the West. It was titled "Targeting the Populations of Countries that Are at War with the Muslims."

Al-Awlaki served as imam at the Dar al-Hijrah mosque in Falls Church, Virginia, a Washington suburb, for about a year in 2001.

The mosque's outreach director, Imam Johari Abdul-Malik, has said mosque members never saw al-Awlaki espousing radical ideology while he was there, and he believes al-Awlaki's views changed after he left the U.S.