Security has been stepped up at the major European gateway of Amsterdam Schiphol on the weekend in response to “indications” of a threat, an official said, causing traffic jams during the busy summer holiday season. Armed police have been conducting extra-security checks at the fifth busiest passenger airport in Europe.

The National Coordinator for Counter-terrorism and Security Policy (NCTV) provided no details of the possible threat, but it said extra security measures were implemented, including vehicle searches.

“There was a (threat) indication related to the airport,” said spokesman Edmond Messchaert. “The increased measures are intended to ensure the safety of people working at the airport and travelers,” according to a Reuters report.

The national threat level in the Netherlands was unchanged at “substantial,” or one notch from the highest. After attacks by Islamist radicals in France, Belgium and Germany, the Netherlands is considered a prominent target.