THE US is bracing for possible revenge terrorist attacks after a CIA-led airstrike in Yemen killed US-born radical cleric and al-Qa’ida figure Anwar al-Awlaki.

The State Department issued a worldwide travel alert yesterday that warned of potential retaliation inside the US and abroad.

“Awlaki’s standing as a pre-eminent English-language advocate of violence could potentially trigger anti-American acts worldwide to avenge his death,” the alert said.

Before the targeted assassination that killed him and six other people two days ago, Awlaki was regarded by Washington as “the most significant risk” to the US because of terrorist plots he inspired.

President Barack Obama called him the leader of “external operations” for Al-Qa’ida in the Arabian Peninsula, a Yemeni-based offsohot of the terrorist network formed by the late Osama bin Laden that is now considered more dangerous than the original.

The State Department travel alert was issued a day after a joint FBI and Homeland Security Department bulletin said Awlaki’s death “could provide motivation for homeland attacks”.

The killing of Awlaki has reignited debate over the legality of targeting US citizens, after President Barack Obama approved the assassination in April last year.

Awlaki was born in the US state of New Mexico in 1971 and ran mosques as an imam in California and Falls Church, Virginia, before leaving the US for Yemen. There he is said to have become radicalised after being jailed, interrogated, and allegedly tortured by Yemeni authorities.

For the assassination order, the Obama administration relied on Justice Department advice and the authority of congress that Awlaki had become a high-level belligerent of enemy forces plotting to kill Americans, based on international law recognising the right of self-defence.

The administration’s position was successfully defended in the US District Court last year in a case backed by the American Civil Liberties Union and brought by Awlaki’s father, who claimed the first targeted assassination against a US citizen breached legal process without his son facing any charges.

The judge ruled it was a “political question” to be decided by the executive branch and congress.

Awlaki was not the leader of al-Qa’ida in the Arabian Peninsula. That title still belongs to Nasir al-Wuhayshi, a former bodyguard of Osama bin Laden who escaped from a Yemeni prison in 2006 and set up the al-Qa’ida offshoot.

But Awlaki was considered extremely dangerous because of his influence in recruiting extremists through internet messages delivered in English and Arabic.

He is alleged to have had email conversations that possibly inspired Nidal Malik Hasan, a troubled US army major charged with murdering 13 people at the Fort Hood military base in 2009.

Awlaki is also accused of giving directions to the alleged “underwear bomber”, Umar Farouk Abdulmutallab, a Nigerian charged with trying to blow up an aircraft before it landed in Detroit on Christmas Day in 2009.

Internet sermons by Awlaki are believed to have inspired Faisal Shahzad, a Pakistani living in the US who unsuccessfully attempted to set off a car-bomb in Times Square, New York, last year, and was arrested as he tried to leave the US.

Awlaki is considered the most high-profile al-Qa’ida figure to be killed since bin Laden in May even if, in tactical terms, the killing of al-Qa’ida deputy Attiyah Abd al-Rahman in Pakistan in late August was more important as an operational blow against the overall terrorist organisation.

Peter King, the Republican House of Representatives homeland security committee chairman, is calling Awlaki’s assassination “as significant as any we can imagine” and more significant than the killing of Bin Laden.

President Barack Obama is calling Awlaki’s death a major blow to al-Qa’ida’s most active branch in Yemen.

From the White House, he said that AQAP remained a dangerous but weakened terrorist organisation.

The demise of Awlaki still leaves the US with significant terrorist targets, including Wuhayshi in Yemen and Bin Laden’s official replacement as al-Qa’ida leader, al-Ayman al-Zarahiri, who is believed to be hiding in Pakistan.

Also killed in a US drone attack on the same day as Awlaki was another US citizen, Samir Khan, who was born in North Carolina and later left for Yemen to edit Inspire, an online al-Qa’ida magazine.