The Guardian is waiting for information on the whereabouts of its correspondent in Iraq, Rory Carroll.
Rory Carroll has been in Iraq for nine months
Mr Carroll, from Dublin, may have been taken by armed men while on assignment in Baghdad, the newspaper said.
Guardian editor Alan Rusbridger said they were "deeply concerned" at his disappearance.
Ireland's Minister for Foreign Affairs, Dermot Ahern, has told the Carroll family his department was ready to offer any assistance required.
Mr Rusbridger appealed to those holding the 33-year-old to release him.
"He is in Iraq as a professional journalist - and he's a very good, straight journalist whose only concern is to report fairly and truthfully about the country," he said.
"We urge those holding him to release him swiftly - for the sake of his family and for the sake of anyone who believes the world needs to be kept fully informed about events in Iraq today."
Mr Carroll, an Irish citizen, was interviewed from Baghdad on Wednesday morning for RTE radio's Pat Kenny Show, about the start of Saddam Hussein's trial.
A few hours later, his family was informed by the editor of the Guardian that he had been "taken".
His father, Joe, told the BBC: "It was something we had been secretly dreading. We were hoping it would never happen."
Mr Carroll said his son had received specialised training for such situations.
"He knew we were worried but he used to reassure us and say it wasn't as dangerous as people outside think and if you observed basic rules of security, you'd be okay," he said.
"We knew he was playing it down for our sake. It was obvious danger.
"He did make it clear to us that he took all the precautions that he thought were necessary."
A graduate of Trinity College, Dublin, Rory Carroll started his career at the Irish News in Belfast, where he was named Northern Ireland young journalist of the year in 1997.
He later joined the Guardian as a home news reporter, and was made South Europe correspondent in 1999.
He was the paper's South Africa correspondent before going to Iraq.
The leader of Fine Gael in the Republic of Ireland, Enda Kenny, said his disappearance was a "major cause of concern".
"I assume the minster for foreign affairs will take a direct and personal interest in this.
"Obviously when anybody is kidnapped it is a cause of concern but as this is an Irish citizen it brings it in to sharper focus for us here."
Mr Carroll has gone missing on the first anniversary of the abduction in Baghdad of Dublin-born aid work Margaret Hassan.