Terrorists hit Brussels Airport and a subway station in Belgium early today, killing at least 30 people by early accounts. ISIS has claimed responsibility.

"Brussels Airport extends its deepest sympathy to the friends and relatives of the victims of the explosions in the departures hall earlier this morning," the airport's PR unit said in a statement.

The airport said two explosions occured in the departures hall at around 8 a.m. this morning, adding emergency and rescue services are on the scene and are offering the necessary assistance.

All flights at the airport have been cancelled.

The attacks, believed to be suicide bombings, put the city of Brussels on lockdown and followed the arrest in Belgium of a key planner of the November 2015 terror attacks on Paris. In the aftermath, US airports and other major transportation hubs have tightened security, as well.

Hotel giant Marriott said all of its guests and employees have been accounted for, adding the attacks have had no direct impact on its hotels. "We are deeply saddened by the tragic events in Brussels and our thoughts and prayers go out to the victims and all those impacted," the company said.

Airline PR operations in the US responded to reassure customers.

Delta said this morning that one of its early flights to Brussels landed safely and was parked remotely at the airport "with the local team working on plans to safely deplane passengers to a secure holding area." A second flight was diverted to Amsterdam. "Delta will be actively working with customers with flights booked to, from or through Brussels to reaccommodate their travel plans," the airline said.

Chicago-based United said its first arrival from Washington to Brussels deplaned normally while its second flight from Newark was rerouted to a remote location.

"The United family is deeply saddened by the tragedies in Brussels earlier today and our thoughts and prayers are with the victims and their familes," United said in a statement.

American Airlines of Fort Worth expressed sympathies and condolensces to those affected by the attacks, noting the explosions did not occur in its check-in area at the airport. Its single flight to Brussels today was canceled.