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.
Last week U.S. Homeland Security Secretary Jeh Johnson said U.S. Homeland Security officials are constantly monitoring world events and evaluating if 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 blasts came days after the arrest of the top suspect in last year's Paris attacks, Salah Abdeslam, in Brussels.
French officials are condemning the Brussels attacks in the strongest terms.
Prime Minister Manuel Valls, speaking after a crisis meeting called by the French president, says "we are at war. We have been subjected for the last few months in Europe to acts of war."
President Francois Hollande says "terrorists struck Brussels but it was Europe that was targeted -- and all the world that is concerned." Hollande also warned that "this war will be long" so sang froid and lucidity are needed.
Paris says it will light the Eiffel Tower in the colors of the Belgian flag. The city's mayor, Anne Hidalgo, described it in a tweet as a measure of "solidarity with Brussels."
German Chancellor Angela Merkel's chief of staff has called for solidarity with Belgium following the Brussels attacks that left scores dead.
Peter Altmaier tweeted Tuesday: "Terrorists will never win."
He added: "Our European values much stronger than hate, violence, terror!"
Russian Foreign Ministry spokeswoman Maria Zakharova says the West's politics of "double standards" have led to terrorist attacks and that frozen diplomatic relations between NATO and Russia have slowed the fight with terrorism.
The Kremlin, meanwhile, has offered its condolences to Belgium and expressed solidarity after the attacks Tuesday that left scores dead.
While Russia and the United States have brokered a fragile peace agreement in Syria, the two countries still disagree on how to tackle terrorist threats posed by the Islamic State group.
Prominent Russian lawmaker Alexei Pushkov also had a jab at Europe and NATO following the Brussels attacks. Pushkov later offered his condolences, but said "it's time for Europe to understand where the genuine threat is coming from and join efforts with Russia."
European Union foreign policy chief Federica Mogherini, fighting back tears, has stopped short a news conference in Jordan after saying that "today is a difficult day," in reference to the Brussels attacks.
Mogherini was wrapping up her opening statement Tuesday at a joint news conference with Jordanian Foreign Minister Nasser Judeh when she was overcome by emotion. When Judeh resumed speaking, she walked over to him, said "sorry" and briefly embraced him. The two then walked off the stage.
Mogherini and Judeh had been speaking for about 16 minutes when the news conference ended abruptly. In her opening remarks, she had talked about the importance of her visit to Jordan, praising the kingdom's stance against militant Islam.
Dutch Prime Minister Mark Rutte says Belgium has "again been hit by cowardly and murderous attacks. Our hearts go out to the victims and next of kin. The Netherlands stands ready to help and support our southern neighbors in any possible way."
Rutte says that "extra alertness is necessary, also in our country. We will take all necessary precautionary measures." Rutte called a meeting Tuesday of his government's Ministerial Crisis Committee to discuss the attacks.
The Dutch anti-terror authority said the country's threat level was unchanged at "substantial." It said extra security measures would be in place at the country's airports and borders.