The kidnappers of American journalist Jill Carroll have threatened to execute her within 72 hours unless US forces free all female Iraqi prisoners.

The ultimatum was sent to the offices of Arab satellite channel al Jazeera, accompanied by a 20-second silent video showing Miss Carroll, 28, looking exhausted and gaunt after 11 days in captivity.

Miss Carroll was seized from the street on January 7 after meeting a prominent Sunni Arab politician at his offices in the west of the capital. Her interpreter was shot dead.

Journalists at Al-Jazeera said the kidnappers identified themselves as members of a previously unknown armed group calling itself the "Brigades of Vengeance". "The kidnappers have set the US government a deadline of 72 hours to free Iraqi women prisoners," the TV station reported.

US forces in Iraq said today that they were currently holding eight women prisoners, after the release of two prominent women scientists last month.

"We have eight females. They are being held for the same reasons as the others, namely that they are a threat to security," said Lieutenant Aaron Henninger, a spokesman for the U.S. military detentions operation. Some 14,000 men are held at Abu Ghraib and other jails on suspicion of insurgent activity.

An Iraqi Justice Ministry official said that there were a number of women among about 7,000 people being held in civilian Iraqi jails under its control, although he did not have an exact figure. All had been convicted of common crimes.

Miss Carroll’s family issued a statement pleading for her release and described her as "an innocent journalist". "We respectfully ask that you please show her mercy and allow her to return home to her mother, sister and family," said the statement.

"She has been welcomed into the homes of many Iraqis and shown every courtesy. From that experience, she understands the hardships and suffering that the Iraqi people face every day."

Miss Carroll, a freelancer working for the Christian Science Monitor, is the 31st journalist to have been kidnapped since the start of the war in 2003, according to Reporters without Borders.

Thousands of angry protesters took to the streets across Pakistan yesterday to condemn an American air strike aimed at al-Qaida's deputy leader, Ayman al-Zawahiri, that left at least 18 people dead.

Up to 10,000 people reportedly protested at rallies in the largest city, Karachi. Many chanted: "Death to America!" Demonstrators demanded the resignation of Pakistan's president, Pervez Musharraf. Smaller protests were staged in other towns and cities, including Islamabad, Lahore, Quetta and Peshawar.

The protests across Pakistan put further pressure on General Musharraf, whose close relationship with the United States has made him unpopular at home.

The rallies followed violent protests by thousands of tribesmen on Saturday in the Bajaur region on the Afghan border where the attack took place. The White House has remained tight-lipped over the missile strike, said to have been carried out by the CIA on Friday, using unmanned drone aircraft.

David Almacy, a White House spokesman, would not even confirm that the attack had been carried out by the US. He said only that "President Musharraf and Pakistan is a valued ally and partner in the war on terror".

However, a Republican senator, John McCain, defended the action yesterday. "We have to go where these people are, and we have to take them out," he said in an interview on CBS television. While expressing sympathy with the anger in Pakistan, he added: "I can't tell you that we wouldn't do the same thing again."