 -- Al Qaeda’s most lethal branch said today that its top cleric, a man with a $5 million American bounty on his head, has been killed in Yemen, as the Arab nation falls deeper into chaos.

The group said in a statement posted online that former Guantanamo Bay detainee Ibrahim al-Rubaysh, also spelled al-Rubaish, was killed in a “crusader strike” over the weekend “after he spent almost two decades carrying out jihad against America and its agents.” The statement did not say who exactly AQAP believed carried out the purported strike.

The U.S. has killed a number of high ranking AQAP leaders in recent years through targeted drone strikes, as part of its sustained counter-terrorism operations in the area.

A senior White House official told ABC News he could not comment on "any specific reports coming out of Yemen," but said, "We continue to actively monitor terrorist threats emanating from Yemen, and we have capabilities postured in the area to address them. As we have in the past, we will take action to disrupt continuing, imminent threats to the United States and our citizens. That remains the priority of this Administration."

Al-Rubaysh is described by the U.S. as a senior “sharia” official in al Qaeda in the Arabian Peninsula (AQAP) who “provides the justification for attacks conducted by AQAP.” He is also accused of being the group’s “senior advisor for AQAP operational planning, and is involved in the planning of attacks.”

Al-Rubaysh was held in Guantanamo Bay prison from 2002 to 2006. AQAP said after he was released he “quickly joined his brethren at AQAP.”

The spiritual leader appears to have played a similar role in AQAP as Anwar al-Awlaki, an American citizen who was a top cleric and recruiter in AQAP before he was killed in a CIA drone strike in September 2011. After his death, the U.S. said al-Awlaki too was involved in the terror group’s external operations.

Al-Rubaysh’s alleged death comes amid widespread violence in Yemen where Saudi Arabia is now leading an Arab coalition effort, supported by U.S. intelligence and logistical support, to repel a Houthi rebel assault against the remnants of Yemeni government forces. Last week the United Nations said the crisis was “getting worse by the hour.” In February, all U.S. government personnel left Yemen as the security situation deteriorated and the American embassy was closed.

AQAP, the Yemen-based branch of al Qaeda, has been previously described by U.S. officials as the affiliate that posed the greatest threat to the American homeland.