Ahmed Said, an official with the National Transitional Council interim government, didn't identify the foreign minister by name, but told media he can confirm that he is in custody.
The most recent foreign minister under Gadhafi was Abdul Ati al-Obeidi.
The development comes as world leaders gather in Paris to outline a democratic future for Libya and also on the 42nd anniversary of the coup that brought Gadhafi to power.
Meanwhile, rebel forces are focusing their efforts to find Gadhafi on three cities which are believed to be loyal to the former strongman, including Sirte, Gadhafi’s birthplace.
Rebels also believe Gadhafi may be in the northwestern city of Bani Walid, which is still controlled by loyalists. Another target is the city of Sabha, a desert outpost in southern Libya.
All three locales have been given an ultimatum to surrender.
The regime is dying, said rebel council spokesman Abdel-Hafiz Ghoga.
Gadhafi's family is trying to find an exit. They only have to surrender completely to the rebels and we will offer them a fair trial. We won't hold negotiations with them over anything.
Meanwhile, an Algerian newspaper El Watan reported that an attempt by Gadhafi to seek asylum in Algeria was rebuffed by President Bouteflika, although members of Gadhafi’s family found refuge there.
Algeria’s foreign minister Mourad Medelci has repeated that Gadhafi isn't in the country and wouldn't be offered asylum.