Homeland Security Secretary Jeh Johnson says there is no "specific, credible intelligence" of any plots against the United States like those carried out in Brussels. 

As a precaution, the Homeland Security Department is stepping up security at major U.S. airports and rail and transit stations around the country.

Johnson says DHS officials are in contact with their European counterparts and monitoring the ongoing situation.

Security officials are also reviewing whether additional screening may be necessary for air travelers coming from Belgium. Belgium is one of 38 countries whose citizens generally do not need a visa to visit the U.S.

Bombs were set off at the main airport and a subway station in Brussels Tuesday morning. At least 30 people were killed and hundreds more were wounded in the attacks.