Florence Aubenas, working for French newspaper Liberation, was first reported missing on Wednesday.
"We don't know what this is," Barnier told LCI television on Friday. "We are worried because we have no news, but we don't know for sure. There are several hypotheses."
Barnier said France was working with the Iraqi authorities to find Aubenas and her Iraqi interpreter Husain Hanun al-Saadi.
The announcement of Aubenas' disappearance comes just weeks after two other French journalists were released on 21 December after being held by fighters in a four-month drama that had gripped the country.
Christian Chesnot and Georges Malbrunot had been held by a group calling itself the Islamic Army of Iraq, which has killed captives in the past.
They said they were released because of France's opposition to the US-led war in Iraq.
Meanwhile, French President Jacques Chirac reiterated on Friday a formal warning to journalists to stay out of Iraq.
The president told journalists at a traditional New Year's reception said he was worried for Aubenas and that Paris would use all possible means to locate her.
Chirac: 'We are mobilising all
means to obtain information'
"Today, we have no news of your colleague and we are worried," he said. "Naturally, we are mobilising all possible means to obtain information and to find her."
The French authorities have formally advised against sending
journalists to Iraq, the president reiterated.
"For everyone, it is a question of responsibility. At the current
time, the security of our correspondents cannot be ensured," he said.