The CIA believes the voice on an audio tape that surfaced Thursday likely belongs to Ayman al-Zawahiri (search), the No. 2 figure in the Al Qaeda (search) terror group, intelligence officials told Fox News on Friday.

But the CIA is unclear exactly when the tape was made. The speaker makes mention of a military operation under way in the Pakistani mountains but that offensive has been in progress for a few weeks now.

The recorded message called Pakistani President Pervez Musharraf (search) a "traitor" and urged people to overthrow his government.

"Musharraf seeks to stab the Islamic resistance in Afghanistan in the back," the speaker said.

"Every Muslim in Pakistan should work hard to get rid of this client government, which will continue to submit to America until it destroys Pakistan."

The pan-Arab satellite channel Al-Jazeera broadcast a seven-minute excerpt from the tape it received Thursday. It made references to the Islamic holy book, the Quran (search), which is known to be al-Zawahiri's style.

Pakistani Foreign Ministry spokesman Masood Khan said Thursday the government had no immediate comment on the purported al-Zawahiri tape. When an al-Zawahiri tape released in September called for Musharraf's overthrow, the government said it would not be deterred in its pursuit of terrorists.

In Washington, Defense Secretary Donald H. Rumsfeld (search) said he heard news reports about the tape and said if it was authentic, the speaker "is clearly an individual who is very high-ranking and is capable of, and has in the past killed innocent men, women and children. And so one has to recognize that."

The tape comes as Pakistani troops are in the second week of a campaign along the Afghan border in South Waziristan, a longtime hiding place and stronghold of Islamic militants from Al Qaeda, Afghanistan's Taliban and their Pakistani supporters.

Apparently referring to the conflict in South Waziristan, the speaker said: "I call on the Pakistani army: you, poor army, what a miserable state Musharraf has put you in ... Musharraf ruins your natural fences — those tribes on the border — by engaging you in a fight with them. Then he removes your nuclear weapons.

"Will you stay silent until Pakistan is divided again?"

The speaker repeatedly named the Pashtun and Baluch border tribes and urged them to close ranks with the Taliban against the Pakistani army.

"Taliban and their supporters are your brothers," the speaker said, "so how can you allow the agents of crusaders and Jews to hurt them?"

He used the word "crusaders" for Americans as Islamic militants often do.

The speaker said such military operations on the borders violated Islamic law by pitting Muslims against each other at America's bidding.

"Every soldier who finds this act to be legitimate is an infidel, according to Islamic law," the speaker said.

After Musharraf spoke hopefully of capturing a "high-value target," there was speculation that al-Zawahiri might be in the rugged border area where local tribes have more power than the Pakistani federal government.

Pakistan Interior Minister Faisal Saleh Hayyat said Thursday that more than 50 terrorists have been killed in the operation. More than 150 suspects have been captured, said Brig. Mahmood Shah, chief of security in Pakistan's tribal areas.

The speaker did not mention Israel's Monday morning assassination of Hamas founder Sheik Ahmed Yassin, which suggests the tape was recorded before then.

Al-Jazeera spokesman Jihad Ballout said the Qatar-based channel received the tape Thursday, but he declined to reveal how. The tape is 17 minutes long.

"Indications are that it is authentic," Ballout said, "the voice, the nuances."

Al-Zawahiri is believed to have provided much of the ideology driving Al Qaeda since his Egyptian Islamic Jihad merged with Usama bin Laden's network in 1998, experts say. The United States has offered a $25 million reward for his capture.

Fox News' Bret Baier and The Associated Press contributed to this report.