The prisoners had been held for several months without charge at Abu Ghraib prison on the outskirts of Baghdad, at Camp Bucca, a US jail in southern Iraq, and at Camp Suse near the Kurdish city of Sulaimaniyah.
"The US military freed all the prisoners this morning," said an Iraqi government spokesman on Sunday, adding that they had been cleared of terror-related charges.
Majed Hameed, an Iraqi reporter for the Dubai-based Al-Arabiya network and the Reuters news agency, was released after four months in US custody, US Army Brigadier-General Mark Kimmitt said during a visit to Al-Arabiya's headquarters in Dubai.
Ali al-Mashhadani, a photographer and cameraman for Reuters, was also freed in the mass release of 500 prisoners from Abu Ghraib prison west of Baghdad, Reuters reported. Al-Mashhadani had been jailed since August.
US-led forces in Iraq are still holding more than 12,000 Iraqis in prisons across the country on suspicion of taking part in the armed operations against US-led troops and US-backed Iraqi troops.
US soldier, civilians killed
In other developments, four Iraqi civilians and a US soldier were killed in a car accident in the northern Iraqi city of Mosul on Sunday, the US military said.
Another US soldier and a civilian were injured in the crash which involved a passenger vehicle and an armoured humvee, the US military said.
"The injured civilian and the two soldiers were taken to a nearby military medical facility, where one soldier was pronounced dead," the military said.
Iraqi police officers are routinely
targeted for killing by fighters
Mosul police officer Fatehi Khodor said the dead civilians were a male driver and three women passengers.
On Saturday, a US marine was found dead on Saturday from an apparent non-hostile bullet wound in Haditha, the US military said.
In other incidents, armed men in Baghdad killed five civilians, two police and a former Iraqi army officer in separate incidents.
Three bodies were found at a sewage plant in southeastern Baghdad.
The body of a criminal intelligence officer who was abducted on Saturday was found in Basra.
The bodies of a police brigadier and his bodyguard were found on a highway between Kirkuk and Beiji
Fighters' unity bid
According to an internet statement posted on Sunday, al-Qaida in Iraq and some other armed groups have set up an umbrella body to coordinate their fight against US-led forces and the Iraqi government.
The Mujahideen Council aims to confront the "Crusaders and their rejectionist and secularist followers who have seized Baghdad", said the statement attributed to al-Qaida, the Army of the Victorious Sect and four lesser known Sunni Muslim groups.
Army of Ansar al-Sunna is one of
the most deadly anti-US groups
The council, which does not include leading anti-US groups such as the Army of Ansar al-Sunna and the Islamic Army in Iraq, said it "welcomes anyone desiring (Islam's) victory to join".
The statement's authenticity could not be verified. It was posted on a website often used by anti-government and anti-US fighters, which subsequently carried messages from the council claiming responsibility for attacks in Iraq in recent days.
"The council also calls on Muslims in Iraq and across the world to join the jihad in Iraq to fight for the victory of religion and to defend the oppressed," it said.
Al-Qaida in Iraq, led by Abu Musab al-Zarqawi, Washington's most wanted man in the country, is one of the main groups fighting US-led forces and the US-backed Iraqi government, which is dominated by Kurds and Shia Muslim Arabs.
Iraq's electoral commmission on Monday will reveal the outcome of its investigation into a raft of complaints of fraud arising out of last month's parliamentary poll, a spokesman said on Sunday.
For their part, foreign monitors have said they will release the conclusion of a separate probe into the election on Thursday.
Initial indications from the December 15 vote showed that Iraq's Shia majority came out on top, but final results have been delayed by allegations of ballot-rigging from Sunni-backed and secular parties.
Final election results have been
delayed by talk of vote-rigging
The electoral commission said it would hold a press conference on Monday afternoon to reveal its decisions on the complaints, the spokesman said.
Abdul Hussein al-Hindawi, a commission member, said on Saturday he expected the results from 150 to 200 ballot boxes to be voided because of fraud.
The commission spokesman said it was not known when final uncertified results would be announced. "That is still under discussion," he said.
The Canada-backed International Mission for Iraqi Elections said it would release a final report on Thursday instead of an interim report, which had been due out on Sunday.
"Because its work has proceeded more quickly than anticipated, it has decided to move directly to the final report," the mission said in a statement.
A positive assessment by the experts will give further credibility to the electoral commission, which has been accused of being biased.