CAIRO (AP) ― Al-Qaida’s Yemeni offshoot on Monday 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.

On Monday, 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 blood of the sheik (al-Awlaki) and his brothers will not go in vain; there are heroes behind him who do not sleep under oppression, and they will retaliate soon,” the group said. “We and the Americans are at war: we get them and they get us, and the end is for those who are patient - they are the ones who will be victorious.”

The strike that killed al-Awlaki also killed a second American, Samir Khan, who edited al-Qaida’s Internet magazine. AQAP said two other militants were also killed.

Al-Awlaki, born in New Mexico to Yemeni parents, was believed to be key in turning al-Qaida’s affiliate in Yemen into what American officials have called the most significant and immediate threat to the Untied States.