Dutch police stepped up security patrols at airports and tightened checks at borders after attacks in neighboring Belgium on Tuesday, March 22, the security agency said, according to Reuters.

Travelers passing through Amsterdam's Schiphol Airport reported delays and a heavy police presence.

Security agencies declined to give details of any further measures taken, but maintained the national threat level at "substantial", one notch below the highest, Reuters said.

Flights were diverted from Brussels to Amsterdam following attacks at the Belgian capital's airport earlier on Tuesday in which at least 13 people are believed killed. Trains heading south to Belgium were subject to indefinite delays, Dutch state railways said.