Al Qaeda's second-in-command is urging Muslims to keep up attacks on the "criminal" United States and has criticised president-elect Barack Obama for promising to back Israel.
The US Homeland Security Department said the message by Ayman al-Zawahri held no indications of an increased threat to the United States
al-Zawahri criticised Senator Obama, whose father was a Kenyan Muslim, for what he described as turning his back on his Muslim roots.
He described the president-elect as a "house slave", contrasting him with 1960s black American Muslim leader Malcolm X.
al-Zawahri also warned Senator Obama of failure should he pursue the policies of President George Bush.
An Obama spokeswoman in Washington said the president-elect's office was not planning to comment.