A previously unknown militant group has released a video of two Western journalists who were kidnapped in the Gaza Strip nine days ago.
Olaf Wiig, 36, and Steve Centanni, 60, of US channel Fox News, were shown telling their families they were in "fairly good health".
A fax from the "Holy Jihad Brigades" to news agencies demanded the US release "Muslim prisoners" within 72 hours.
The US rejected the demands, and called for the men's unconditional release.
The two men were kidnapped in Gaza City on 14 August.
The video was sent to the Qatar-based TV news channel al-Jazeera TV.
The footage shows the two men seated on the ground side-by-side and cross-legged, in an apparently darkened room, dressed in tracksuits.
Call for pressure
"We're in fairly good condition, we're alive and well," Mr Centanni, a US citizen and a Fox News reporter, says in the video.
"Just want to let you know I'm here and alive and give my love to my family and friends and ask you to do anything you can to try to help us get out of here."
He adds that the two men have been given access to clean water, showers, toilets, food and clothing.
Mr Wiig, a cameraman from New Zealand, asks his family and supporters to apply pressure for their release on the Palestinian government in Gaza and the West Bank.
"To my family, I love you all. Please don't worry, I'll do all the worrying for us," Mr Wiig says.
The group also issued pictures of the journalists' identity cards, and, in a separate statement faxed to news agencies, called for a prisoner release within three days.
"We will give you one chance that will not be repeated - the liberation of Muslims detained in American prisons in exchange for the detainees in our hands," the statement read.
"We give you a deadline of 72 hours starting from today at noon to decide."
The statement, which was written in strongly religious language and quoted from the Koran, did not say what would happen to the two journalists if the US did not comply.
A US state department spokesman rejected the demand and condemned the kidnapping.
The militants also showed pictures of the journalists' ID cards
"We do not make concessions to terrorists and we continue to call for the release of these journalists unconditionally," he said.
Fox senior vice president John Moody said the company was encouraged that the journalists appeared to be alive and well.
"We trust that the abductors understand they are responsible for Steve and Olaf's welfare and safe return. We ask for their immediate release," he said in a statement.
The Palestinian Prime Minister, Ismail Haniya, of the militant group and ruling party Hamas, re-iterated his call for the journalists' to be freed.
"Our demand for the people behind the kidnapping to release the journalists still stands, because... this act contradicts the habits and customs of our people," he said.
There had been no word on the journalists since they were seized from their vehicle near the Palestinian security services' headquarters in Gaza City.
The crew's Palestinian driver told security officials their car had been stopped in Gaza City .
Mr Wiig's wife, Anita McNaught, has appealed for his release
He said masked gunmen ordered the men into another vehicle and they were driven away.
The BBC's Nick Thorpe in Jerusalem says several factors make this kidnapping more serious than several previous abductions of foreign journalists and aid workers in Gaza in recent months.
The length of time the journalists have been held is longer than in previous cases, and also their fate has now been linked to the question of prisoners and thus to the wider conflict in the Middle East, our correspondent says.
The fate of Israeli corporal, Gilad Shalit, who was seized by Palestinian militants who tunnelled into Israel on the 25th of June, remains unknown.
Two Israeli soldiers captured by Hezbollah militants just inside Israel's northern border have also still not been returned, despite 34 days of fighting.
Correspondents say the previously unknown group could be an offshoot of one of the existing Palestinian militant groups, but it could also be a group that identifies itself more with the agenda and aims of al-Qaeda.
They point to the religious language in the video, and the fact that it contains few references to the Palestinian cause, but refers to the evil powers which it says are fighting Islam.