Two members of the Medecins Sans Frontieres medical team in Baghdad have been missing since last Wednesday.
The remaining four members of the team have been pulled out of the 250-bed Al Kindi General Hospital where they have been working, but are still in Iraq.
"We now have to assume that Iraqi officials are holding the two men," said Petrana Nowill, spokeswoman at MSF.
The missing men from the six-strong team are support staff Francois Calas, of France, and Ibrahim Younis, of Sudan. Others in the group include a Norwegian doctor, an Austrian anaesthetist, an Italian surgeon and support staff from Algeria, the Sudan and France.
More stable conditions in southern Iraq have prompted several aid charities to send in personnel across the border. Two UK staff from Cafod were sent into the Iraqi port of Um Qasr last week.