Security tightened in Houston after Belgium attacks

Security at airports in Houston, the fourth largest city in the US, has been heightened after the terror attacks in Brussels, officials said.

Houston Mayor Sylvester Turner on Wednesday said his city faces no specific threats, but police have stepped up patrol in ticket lobbies and parking garages of the airports, and drivers are not allowed to linger in passenger pick-up and drop-off lanes, Xinhua reported citing local TV station KHOU 11.

The mayor said these precautions were expected to continue through the National Collegiate Athletic Association (NCAA) Final Four events -- Men's Basketball Championship scheduled for April 2-4.

"We always assume for the worst case scenario with respect to any of the events, especially the NCAA. So we were already ramped up to make sure that we keep people safe," Turner said.

The biggest risk in the US was "homegrown radicalised and violent extremists as opposed to actual ISIS members coming over", said Dennis Storemski, Houston's director of public safety and homeland security. "The main thing is to try to prevent it."

At least 34 people were killed and around 200 others injured on Tuesday in a series of explosions at Zaventem airport and Maelbeek metro station in Brussels.