Al-Qaida's second-in-command urged Muslims to continue attacks on "criminal" America and slammed U.S. president-elect Barack Obama for vowing to back Israel during his campaign.

Ayman al-Zawahri warned Obama that he would fail if he followed the policies of President George W. Bush, according to an audio tape published on Wednesday by the SITE Institute, a U.S. organization that monitors Islamic militant groups.

"America, the criminal, trespassing crusader, continues to be the same as ever, so we must continue to harm it, in order for it to come to its senses," Zawahri said in a message to Muslims across the world.

"Its (America's) criminal, expansionist Crusader project in your lands has only been neutralised by the sacrifices of your sons, the mujahideen."

Zawahri also criticised Obama for what he described as turning his back on his Islamic roots.

"The Muslim nation received with extreme bitterness your hypocritical ... stance towards Israel," he said. "You were born to a Muslim father, but you chose to stand with the enemies of Muslims."