Anwar al-Awlaki, a radical cleric and one of the most influential al-Qaeda operatives wanted by the U.S., was killed Friday in an airstrike in northern Yemen, authorities said.

President Barack Obama has described this as another significant milestone in efforts to defeat al Qaeda.

This is further proof that al Qaeda and its affiliates will have no safe haven anywhere in the world, said Obama.

Yemen's embassy in Washington said Awlaki had been killed some miles from the town of Khashef in the northern province of Jawf, adjacent to Saudi Arabia. At least four people were killed in the operation, Yemeni officials said. These included, Samir Khan, who was an editor and illustrator for Inspire, the online magazine of al Qaeda in the Arabian Peninsula.

Both U.S. and Yemeni officials characterized the operation as a joint U.S.-Yemeni effort. The key tip that led to the strike on Mr. Awlaki came from Yemeni intelligence, according to U.S. officials.

U.S. drone aircraft had targeted Awlaki in May but missed. The United States has stepped up drone strikes in Yemen to try and keep al Qaeda off balance and prevent it from capitalizing on the strife and chaos gripping the country.

Awlaki, born in New Mexico, preached at mosques in the United States attended by some of the hijackers of the Sept. 11, 2001 attacks by al Qaeda, whose leader, Osama bin Laden, was killed in a U.S. raid on his hideout in Pakistan in May.

Anwar al-Awlaki's value to al-Qaeda and the risk he posed to the U.S. was embedded less in the operational capacity of the terror network and more in his role as a charismatic spokesman for a movement that was frenziedly trying to connect with Muslims.

Al-Awlaki is credited with inspiring a string of recent attacks and attempted assaults on the U.S., including the 2009 failed Christmas Day bombing of an airliner over Detroit.