President Obama said Tuesday the United States “stands in solidarity” with the people of Belgium and will do “whatever is necessary” to bring the attackers behind the Brussels bombings to justice.

“We can and we must defeat those who threaten the safety and security of the people in the world,” Obama said before delivering from Havana a prepared televised address to the Cuban people.

He added that the attacks are another reminder that the world “must unite” against the “scourge of terrorism.”

House Speaker Paul Ryan criticized Obama’s trip to Cuba in the wake of the attacks, calling the trip “ironic” and a waste of time. 

Obama was briefed Tuesday morning on the explosions at the Brussels airport and subway system that claimed the lives of 31 people and left more than 180 injured. The president spoke with Belgian Prime Minister Charles Michel to offer his condolences. 

Secretary of State John Kerry said in a statement that the U.S. was working "to determine the status of all American citizens in Brussels." The embassy there issued a statement telling Americans to stay where they are and "take the appropriate steps to bolster your personal security."

At least one of the attacks was believed to be caused by a suicide bomber, and Belgium raised its terror alert to its highest level.

Rep. Adam Schiff, D-Calif., the ranking member of the House intelligence committee, said the attacks "bear all the hallmarks" of an Islamic State group coordinated or inspired attack. His staff said he received a preliminary briefing Tuesday from U.S. officials. Schiff says it's unclear if encrypted communications played a role in the attacks but noted that the Brussels attacks occurred despite the city being under constant vigilance.

The U.S. Department of Homeland Security said it was closely monitoring the unfolding events and "would not hesitate to adjust our security posture, as appropriate, to protect the American people."

DHS reiterated that members of the public should report any suspicious activity in their communities to law enforcement authorities.

Attorney General Loretta Lynch was also briefed on the attacks, Justice Department officials in Washington said. They said the Justice Department and the FBI was coordinating with other U.S. government agencies, as well as with Belgian counterparts.

Last week U.S. Homeland Security Secretary Jeh Johnson said Homeland Security officials constantly monitor world events and evaluate whether there is a need to either publicly raise the nation's security posture or issue another bulletin via the government's National Terror Advisory System.

Such a bulletin was issued in December advising the public that federal law enforcement was concerned about the possibility of homegrown violent extremists and terrorist-inspired individuals.

The Associated Press contributed to this report.