Security forces in Yemen have killed Said al-Shihri, described as the second-in-command of a regional branch of Al Qaeda, senior U.S. and Yemeni officials say. Al-Shihri's death is a major blow to the militant group.

The U.S. officials said Saeed al-Shihri was killed, but could not confirm any U.S. involvement in the airstrike Monday that Yemeni leaders say killed the terrorist and five others.

Yemeni officials said the missile that killed al-Shihri was believed to have been fired by a U.S.-operated, unmanned drone aircraft. The U.S. doesn't usually comment on such attacks although it has used drones in the past to go after Al Qaeda members in Yemen.

Al-Shihri is a former inmate of Guantánamo Bay who was released to Saudi Arabia in 2007 and put through a Saudi rehabilitation program for militants, according to the Guardian.

A Yemeni security source also told the Guardian that another Saudi and an Iraqi national were among the others killed.

The two U.S. officials spoke on condition of anonymity because they were not authorized to release the information to the news media.

Al Qaeda's Yemen branch is seen as the world's most active, planning and carrying out attacks against targets in and outside U.S. territory.

Al-Shihri's death would amount to a major breakthrough for U.S. efforts to cripple the group in Yemen, which is considered a crucial battleground with the terror network. The impoverished nation on the southern tip of the Arabian Peninsula is on the doorstep of Saudi Arabia and fellow oil-producing nations of the Gulf and lies on strategic sea routes leading to the Suez Canal.

The group took advantage of the political vacuum during unrest inspired by the Arab Spring last year to take control of large swaths of land in the south. But the Yemeni military has launched a broad U.S.-backed offensive and driven the movement from several towns.

Al-Shihri would be the latest in a series of Al Qaeda figures killed in drone strikes, including U.S.-born Anwar al-Awlaki, who had been linked to the planning and execution of several attacks targeting U.S. and Western interests, including the attempt to down a Detroit-bound airliner in 2009 and the plot to bomb cargo planes in 2010.

Officials said the Al Qaeda in Yemen deputy was killed as he left a house in the southern Hadramawt province with his five companions.

Al-Shihri, who is believed to be in his late 30s, fought in Afghanistan and spent six years in the U.S. military prison at Guantanamo Bay, before being released and going through Saudi Arabia's famous "rehabilitation" institutes, an indoctrination program that is designed to replace what authorities in Saudi Arabia see as militant ideology with religious moderation.

But he headed south to Yemen upon release and became deputy to Nasser al-Wahishi, the leader of Al Qaeda in the Arabian Peninsula, as the terror network's Yemen branch is formally known. Al-Wahishi is a Yemeni who once served as Osama bin Laden's personal aide in Afghanistan.

Al Qaeda in Yemen has been linked to several attempted attacks on U.S. targets, including the foiled Christmas Day 2009 bombing of an airliner over Detroit and explosives-laden parcels intercepted aboard cargo flights last year.

Unlike other Al Qaeda branches, the network's militants in Yemen have gone beyond the concept of planting sleeper cells and actively sought to gain a territorial foothold in lawless areas, mainly in the south of Yemen, before they were pushed back by U.S.-backed government forces after months of intermittent battles.

The Yemen-based militants have struck at Western interests in the area twice in the past 12 years. In 2000, they bombed the USS Cole destroyer in Aden harbor, killing 17 sailors. Two years later, they struck a French oil tanker, also off Yemen.

U.S. drone strikes have intensified in Yemen in recent month, killing several key Al Qaeda operatives.

Samir Khan, an Al Qaeda propagandist, also was killed in a drone strike last year. Last October, al-Awlaki's son was among nine killed in an airstrike. The nine also included Egyptian-born Ibrahim al-Banna, identified by Yemeni authorities as the media chief of the Yemeni branch of the Al Qaeda.

The Associated Press contributed to this report.