Al-Qaida's Yemeni offshoot confirmed the killing of U.S.-born militant cleric Anwar al-Awlaki late last month and vowed to avenge the prominent propagandist's death.

The 40-year-old al-Awlaki, who died in a Sept. 30 U.S. drone strike in the mountains of Yemen, was the most prominent al-Qaida figure to be killed since Osama bin Laden's death in a U.S. raid in Pakistan in May. He had been in the U.S. crosshairs since his killing was approved by President Barack Obama in April 2010 – making him the first American placed on the CIA "kill or capture" list.

Al-Qaida in the Arabian Peninsula said in a statement posted on Islamist extremist websites that al-Awlaki was killed by an American airstrike, along with three other militants, according to the SITE Intelligence Group, which monitors extremist websites. AQAP, which has become the most active al-Qaida branch in recent years, vowed to strike back, The Huffington Post reported.

Known as an eloquent preacher who spread English-language sermons on the internet calling for "holy war" against the United States, al-Awlaki's role was to inspire and – it is believed – even directly recruit militants to carry out attacks.