Belgium is asking Greece to extradite one of the suspects arrested there over a possible link with an alleged militant Islamist plot against Belgian police.
Earlier, Belgium had said there was no connection between those detained and the alleged plot, the BBC reported.
The federal prosecutor's office now says "further analysis" prompted the latest request to Athens.
Europe is on high alert after gunmen killed 17 people in Paris last week.
"Further analysis of the elements of our investigation gave us enough reasons to ask for the extradition of one of the persons that were arrested yesterday by the Greek authorities," the Belgian federal prosecutors' office said in a statement.
It is not clear if the detainee sought is the one that was originally said to match the description of Abdelhamid Abaaoud, the alleged leader of the cell reportedly planning to kill police officers.
More than 20 people have been arrested in Belgium, France and Germany.
For the first time in 35 years, troops are on the streets of Belgium guarding sensitive targets.
More than 150 have been patrolling potential targets for militants, including the US embassy, synagogues and the Jewish museum.
Defence officials said the nation's security threat level would remain raised at three - the second highest level - for at least a week.