Armed military police launched a security sweep at Rotterdam airport on Thursday after receiving an anonymous tip-off about a possible terror threat, AFP reports.
The heavily-armed Dutch police in black, carrying machine guns and wearing bulletproof vests, could be seen in television images checking cars and buses arriving at the airport.
The military police and the normal police forces "immediately launched an investigation" after they "received an anonymous message indicating a possible threat at Rotterdam The Hague Airport," the police said in a statement.
"Certain extra security measures, both visible and invisible, are being put into place," and Rotterdam mayor Ahmed Aboutaleb was being kept informed about developments, it added.
Flights did not appear to have been affected in and out of the airport, which lies about six kilometers (three miles) north of Rotterdam, the busiest port in Europe.
The Netherlands has been on high alert since the Paris attacks a year ago, and on Monday the Dutch counterterrorism agency warned that extremist fighters returning from Iraq and Syria amid an offensive against ISIS could increase the risk of a terror attack here.
Earlier this year a drunk Polish man sparked a late night scare at Amsterdam's Schiphol airport by falsely claiming to have a bomb in his backpack.
Police partially evacuated the airport -- one of Europe's busiest travel hubs -- during that incident, which came just three weeks after the March 22 attacks on the Brussels airport and metro left 32 people dead and hundreds wounded.