Security has been stepped up at US airports and transit systems after the airport and subway bombings in Brussels, though Obama administration officials say there is no specific or credible intelligence of a copycat attack in the country.
"At present, we have no specific, credible intelligence of any plot to conduct similar attacks here in the US," the US Homeland Security Secretary Jeh Johnson said.
"That said, we remain very focused on the threat posed by lone terrorist actors who may lack direct connection to a foreign terrorist organisation; we are concerned that such radicalised individuals or small groups could carry out an attack in the Homeland with little warning," he said in a statement.
"We also remain very engaged in the effort to identify and disrupt foreign terrorist fighters who may seek to travel to or from the US," Johnson said.
The FBI has been in communication with state and local law enforcement, to share the latest about what they know of attacks in Belgium, he said.
"Our personnel have an excellent working relationship with Belgian authorities, and we continually receive information about the attacks and those who may have been involved.
"Since the Paris attacks in November, we have enhanced information sharing about potential terrorist threats with both Belgian and French authorities, and we will continue this effort," he said.
Though the US does not require Belgian citizens to have a visa to travel here for business or tourism purposes, both the Transportation Security Administration (TSA) and Customs and Border Protection (CBP) have procedures in place to identify and prevent travel here from Belgium by individuals of suspicion, Johnson said.
All travelers arriving in the US are vetted against the US Terrorist Screening Database, regardless of whether they arrive with a visa or an Electronic System for Travel Authorization (ESTA), he said.
As a precautionary measure, TSA is deploying additional security to major city airports in the US, and at various rail and transit stations around the country.
TSA is also working closely with state and local law enforcement, airport and transit authorities, and the aviation industry in order to augment that security, Johnson said.
Meanwhile, US President Barack Obama in a proclamation ordered for the US national flag to fly at half-staff to honour the victims of the Brussels terrorist attack.
