Terror attacks in Brussels, Belgium left more than 30 people dead at the city's airport and a subway station.

Officials say three explosions struck Brussels during the Tuesday morning rush hour. Two blasts hit the international airport. About an hour later, another explosion struck a metro station.

Belgian authorities released video footage of a man who was with the two apparent suicide bombers ahead of the attack. The video shows a man in a light-colored coat pushing a luggage cart through the Brussels airport beside the two suspected bombers.

In Havana, President Barack Obama briefly commented on the terror attacks in Brussels before moving ahead with his prepared speech to the people of Cuba.

"We will do whatever is necessary to support our friend and ally, Belgium, in bringing to justice those who are responsible," he said.

The terrorist group known as the Islamic State, or ISIS, has claimed responsibility.

The explosions come just a few days after the prime surviving suspect in the Paris attacks, Salah Abdeslam, was arrested in Brussels.

U.S. security officials say they're on high alert for the possibility of copycat attacks in the U.S., carried out by homegrown violent extremists; attacks similar to the one in San Bernardino last year.

The director of the FBI says his agency is investigating ISIS suspects in all 50 states. Authorities have already arrested 15 people in New York who are accused of having ties to the group.

The Islamic State considers the U.S. its top enemy, and Rep. Michael McCaul, the chairman of the House Homeland Security Committee, told Time Warner Cable News that the group is determined to attack major U.S. cities.

“I think it’s New York and Washington, D.C. that are more the targets that they have talked about extensively in the intelligence briefings that I get," McCaul said.

The president continues to call for international unity in the fight against terror.

"We can - and will - defeat those who threaten the safety and security of people all around the world."

Geoff Bennett also contributed to this report.