Enhanced security measures at Amsterdam's Schiphol airport are to remain in force until further notice after it received a warning against a possible terror threat, a military police spokesperson said Monday.

"The measures will remain in force" until the mayor of Harlemmermeer decides otherwise, said the spokesperson to Dutch news agency ANP.

The Dutch TV channel NOS said on Saturday that the warning could refer to a suspected terrorist attack, while newspaper De Telegraaf noted that the nature of the warning remained unclear and that the measures were of a preventive nature.

Since Saturday, Dutch military forces have controlled vehicle access to the airport, and some in-depth car checks are taking place, according to witnesses contacted by NOS.

The Haarleemmermeer city hall in the town where the airport is located and the National Coordinator for Terrorism and Security declined to comment on the exact nature of the warning.

Holland has been on heightened security alert since 2013.

Apart from car security checks, there has been an increase in police patrols inside the airport and nearby, plus other "stealthy" measures, according to NOS. EFE