Al-Qaida says it has picked Egyptian-born Ayman al-Zawahri to succeed Osama bin Laden as head of the terror network.

Al-Qaida posted the announcement on an Islamist website. The group vowed to immediately continue its pursuit of jihad, or holy war, against the United States and Israel and called them “apostate invaders.” Al-Qaida said it would continue its fight “until all the invading armies leave the land of Islam.”

Zawahri, a surgeon who turns 60 next week, had been bin Laden's long-time deputy. Terrorism experts often consider him to be al-Qaida's main strategist and operational organizer. Many analysts assumed he would take over al-Qaida's leadership after U.S. commandos raided bin Laden's Pakistani hideout last month and killed him.

Zawahri is believed to be operating from somewhere in the mountainous region near the Pakistan-Afghanistan border. An American counter-terrorism official, John Brennan, said this week that the U.S. is hunting him.

A U.S. State Department spokeswoman reacted to word of al-Qaida's leadership change by saying, “it barely matters.” Victoria Nuland called al-Qaida a “bankrupt ideology.”

In May, U.S. presidential counterterrorism adviser John Brennan called Zawahri an uncharismatic figure with many detractors within the militant group.

Zawahri comes from an upper middle class family of doctors and scholars, but became involved in radical Islam as a teenager. While earning a medical degree, he helped form the Egyptian Islamic Jihad militant group.

After bin Laden was killed, Zawahri lauded the mastermind of the 2001 terrorist attacks on the U.S. Zawahri said that bin Laden “went to his God as a martyr.”

Zawahri continued al-Qaida's steady condemnation of the West. He also has criticized Arab states al-Qaida considers to be godless and too closely allied with the U.S. He pledged allegiance to the leader of the Afghan Taliban, Mullah Omar, and described him as the “Emir of Believers.”

In a recording, he contended that NATO's aerial attacks on troops loyal to Libyan leader Moammar Gadhafi are designed for a Western takeover of the country's oil wealth.

But he also praised his fellow Egyptians for overthrowing President Hosni Mubarak, and voiced support for the Syrian uprising against the authoritarian rule of President Bashar al-Assad.