President Barack Obama, traveling in Cuba, was briefed Tuesday morning on the Brussels attacks that killed dozens of people. The White House said U.S. officials were in contact with Belgian officials about the explosions at the Brussels airport and subway system.

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.

The U.S. Department of Homeland Security was closely monitoring the unfolding events in Brussels, officials said Tuesday. The department said it “would not hesitate to adjust our security posture, as appropriate, to protect the American people.”

It 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.

In Brussels, the U.S. Embassy recommended sheltering in place and warned U.S. citizens there to monitor local developments and take “appropriate steps to bolster your personal security.”

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.

(AP)