WASHINGTON — The Obama administration stepped up security at major transit hubs across the country after Tuesday’s airport and subway bombings in Brussels, as top U.S. intelligence officials warned of copycat attacks at home.

Homeland Security Secretary Jeh Johnson said there is no “specific, credible intelligence” pointing to a similar plot in America, but he said the Transportation Security Administration would deploy additional security at major airports and rail stations in different cities. Officials also reviewed additional security measures for travelers from Belgium, among more than three dozen countries whose citizens generally don’t need a visa to enter the U.S.

The attacks in the Belgian capital underscored the growing threat posed by the Islamic State group on both sides of the Atlantic. The bombs in Brussels’ airport and subway locked down the European Union’s capital just a few months after attacks shocked Paris and San Bernardino, Calif.

The Homeland Security Department said it could further enhance security measures “as appropriate, to protect the American people.” It urged Americans to report any suspicious activity to police.

Johnson said last week officials were monitoring world events while evaluating whether to raise the nation’s security posture.