An American-born cleric killed in Yemen played a "significant operational role" in plotting and inspiring attacks on the US, officials said, as they disclosed detailed intelligence to justify the killing of a US citizen.

Anwar al-Awlaki was killed early today in a strike on his convoy carried out by a joint operation of the CIA and the US Joint Special Operations Command. Al-Awlaki had been under observations for three weeks while they waited for the right opportunity to strike.

Following the strike, a US official outlined new details of al-Awlaki's involvement in anti-US operations, including the attempted bombing on December 25 2009 of a US-bound aircraft. The official said al-Awlaki specifically directed the man accused of trying to bomb the Detroit-bound plane to detonate an explosive device over US airspace to maximise casualties.

The official also said al-Awlaki had a direct role in supervising and directing a failed attempt to bring down two US cargo aircraft by detonating explosives concealed inside two packages posted to the US. The US also believes Awlaki had sought to use poisons, including cyanide and ricin, to attack westerners.

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

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

The US reports of al-Awlaki's death came after the government of Yemen reported that he had been killed about five miles from the town of Khashef, some 87 miles from the capital, Sanaa.

The air strike was carried out more openly than the covert operation that sent Navy SEALs into bin Laden's Pakistani compound.

Counterterror cooperation between the United States and Yemen has improved in recent weeks, allowing better intelligence-gathering on al-Awlaki's movements, US officials said. The ability to track him better was a primary factor in the success of the strike, they said.

Al-Awlaki's death is the latest in a run of high-profile killings ordered by President Barack Obama. Still, the killing raises questions that the death of other al-Qa'ida leaders, including bin Laden, did not.

Al-Awlaki is a US 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.

US officials have said they believe al-Awlaki inspired the actions of Army psychiatrist Major 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.

Yemen's defence ministry said that another American militant, Samir Khan, who produced an English-language al-Qa'ida web magazine, died in the same US air strike that killed al-Awlaki.

Khan, in his 20s, was an American of Pakistani heritage from North Carolina who produced Inspire, an English-language web magazine which spread al-Qa'ida ideology and promoted attacks against US targets, even running articles on how to put together explosives.

In one issue. Khan wrote that he had moved to Yemen and joined al Qaida's fighters, pledging to "wage jihad for the rest of our lives."