Majid Hameed, who worked for al-Arabiya in the city of Ramadi and also worked on a freelance basis for Reuters, was arrested along with several other men at the funeral of a relative.
"Al-Arabiya considers the detention of its reporter to be a blatant violation of internationally recognised guidelines and regulations that safeguard press freedoms and freedom of expression," Dubai-based al-Arabiya said in a statement.
Reuters said it was deeply concerned that another Iraqi journalist was being held without charge by US forces and called for any accusations against Hameed to be made public.
"It is imperative that journalists should not be held in Iraq unless there are proper and public charges to justify their detention," a Reuters spokeswoman said.
Al-Arabiya said the US military had confirmed that Hameed was being held at a US facility in western Anbar province. It said US military officials had told Al-Arabiya that Hameed was detained on Saturday, although some reports said he had been arrested last Thursday.
Al-Arabiya said it was outraged at "the time it took relevant US authorities to acknowledge the whereabouts of Hameed".
In a televised interview with Al-Arabiya, US military spokesman Captain Eric Clark said: "We have credible evidence that led us to apprehend your correspondent."
He declined to specify the evidence against Hameed, saying an investigation was under way.
US military has been criticised for
detaining civilians without trial
Several Iraqi journalists are being held without charge by US forces, including two freelance cameramen working with Reuters.
One of them is Ali Omar Abrahem al-Mashhadani, also from Ramadi, who was arrested last month and is being held in Abu Ghraib prison without access to lawyers or relatives.
Reuters has demanded that the US military specify the charges against its detained journalists or release them. International media rights groups have also condemned the detention without trial of journalists in Iraq.
According to accounts from relatives, both the detained Reuters cameramen were arrested after troops viewed images on their cameras.
Reuters says that as part of their work as independent journalists in Iraq, the cameramen have filmed images of conflict, and this does not imply any links with fighters.
"It is imperative that journalists should not be held in Iraq unless there are proper and public charges to justify their detention"
The US military says arrested journalists in Iraq cannot expect any special consideration from the authorities and will be treated like any other detainee.
US forces have the power to detain Iraqis without trial if they believe they pose a security threat.
In an interview with Reuters a week ago, Iraqi Justice Minister Abdul Hussein Shandal condemned US forces for holding Iraqis for long periods without trial and said the media must have special protection to report on all sides in the conflict.
Among the other Iraqi journalists in US custody is a cameraman for US network CBS who was detained in April in the northern city of Mosul and is still being held without charge.
An Iraqi editor was meanwhile killed outside his Mosul home on Tuesday, the third journalist murdered in Iraq in four days, the Committee to Protect Journalists reported on Wednesday.
Fakher Haidar (R) was found shot
dead on 19 September in Basra
The New York-based group said Firas Maadidi, 40, was the
Mosul bureau chief for As-Saffir and chief editor of the local
He was killed after being shot six times, including two shots to the head, As-Saffir deputy editor Slayhe Jowiree told CPJ.
Maadidi died hours later in hospital, the statement said.
On Saturday, Hind Ismail, a reporter for As-Saffir, was
killed by a single bullet to the head. Fakher Haider, an Iraqi journalist who worked for The New York Times, was abducted on Sunday night and found dead on Monday.
Since the Iraq conflict began in March 2003, 56 journalists
have been killed. CPJ said two-thirds of those killed have been Iraqi journalists.