Al-Qaeda's longtime No. 2 leader, Ayman al-Zawahiri, has succeeded the slain Osama bin Laden as head of the terrorist network, according to a statement by the group on Thursday.

Al-Qaeda said that under Zawahiri's leadership, it would continue its holy war against the United States and Israel.

Zawahiri, who's turning 60 this month, is a surgeon from a prominent Egyptian family. He worked with Osama bin Laden for decades and is credited for al Qaeda's use of bombings and independent terror cells.

"We seek with the aid of God to call for the religion of truth and incite our nation to fight ... by carrying out jihad against the apostate invaders ... with their head being crusader America and its servant Israel, and whoever supports them," said the statement.

Al-Qaeda, an international terrorist network, is considered the top terrorist threat to the United States. The group is wanted for its Sept. 11, 2001, attacks on the World Trade Center and the Pentagon.

bin Laden, founder of al-Qaeda, in May was shot and killed inside a secured private residential compound in Abbottabad, Pakistan, by U.S. Navy SEALs in a covert operation ordered by Obama. Shortly after his death, bin Laden's body was buried at sea.

While the killing of bin Laden is a major blow to the terrorist organization, many say the Al-Qaeda remains a dangerous threat with its networks spread all over the world.

Zawahiri is now the United States' most wanted man.