French authorities, citing the risk of an imminent threat, have said they are keeping five men in custody without charge amid a probe into a suspected new plot to attack the country.

The suspects were believed to have taken orders from Syria, and used a mobile phone and computers to search for locations that officials said might have been potential attack sites.

Security was tightened at the headquarters of France's criminal investigations police, reportedly among the locations studied.

The five were arrested on Sunday in Strasbourg and Marseille, and should have been charged or released early Thursday. Authorities used an exceptional recent anti-terrorism measure to extend their custody up to two more days, according to a judicial official.

The official said the exceptional move was based on the threat of an imminent terrorist act and the need for further information from international partners.

A security official said investigators have determined that the suspects used the phone and computers to search locations, including an amusement park - but said it was too early to tell whether these were attack targets.

The Paris prosecutor's office and the interior ministry would not comment on a report by BFM television that the suspects were plotting a possible attack on December 1 and had studied potential targets including Disneyland Paris.

Disneyland Paris said in a statement that the safety and security of its guests is "our number one priority", adding: "We work closely with state and local authorities, and constantly review the security measures we have in place."

France remains under a state of emergency imposed after Islamic State attacks in Paris in November 2015 that killed 130 people.