Arabic television Al Jazeera has aired a purported new tape of senior al Qaeda leader Ayman al-Zawahri in which he called on Pakistanis to overthrow their government.
"I call on Muslims in Pakistan to get rid of their government which is working for Americans," said the voice on the tape, which sounded like previous recordings of Zawahri, the right-hand man of Osama bin Laden. Americans believe Zawahri played a key role in the September 11th, 2001 attacks.
Pakistan's President Pervez Musharraf narrowly escaped two assassination attempts in December blamed on Muslim militants.
Pakistan, a key US ally in the "war on terror", has been waging a bloody campaign in the last fortnight to root out al Qaeda fighters on its Afghan border in tandem with a US sweep on the other side of the frontier.
Pakistani officials said last week the fierce defence the militants were putting up suggested they were protecting a "high-value target", perhaps Zawahri, but the military later said that was unfounded "guesswork".
Islamabad has faced strong opposition demands to stop the offensive in the lawless South Waziristan tribal region by some 5,000 troops, but says it will go on until all militants were eliminated. Scores of people have been killed.