“We all stand together with our Ally Belgium on this dark day,” Stolenberg’s statement said. “This is a cowardly attack. An attack on our values and on our open societies. Terrorism will not defeat democracy and take away our freedoms. ”

A spokesman for NATO’s Supreme Headquarters Allied Powers Europe (SHAPE), about 40 miles southwest of Brussels near the town of Mons, said the organization is closely monitoring the situation and “immediately adapted its security posture in line with the Belgian government’s decision to raise the national alert at its highest level.

“SHAPE takes matters of security very seriously and security staff constantly assess the security environment and adjust our security posture appropriately for the conditions to ensure the security of the installation and our personnel,” said the spokesman, Lt. Col. Christophe Cuny.

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The previous NATO secretary general, Anders Fogh Rasmussen, said in a tweet that “we must all rally around Belgium in the fight for freedom.”

Gen. Philip M. Breedlove, the chief of European Command, announced a temporary ban on non-essential travel to Brussels. Travel by service members to the city while on leave or liberty is prohibited until further notice. Those who must travel there for work must first get approval from a general officer. A similar ban was put in place following the attacks in Paris in November, and rescinded within about two weeks.

The U.S. Embassy in Brussels warned on Tuesday that it recommended Americans shelter in place and avoid public transportation. Its alert status was updated to reflect an imminent or ongoing attack.

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U.S. Defense Secretary Ashton B. Carter said at the outset of a House Armed Services Committee on Tuesday that the attacks in Belgium show again that the United States must destroy the Islamic State militant group in Iraq and Syria.

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Attempts to reach NATO quarters in Brussels were not initially successful Tuesday. It is based southwest of Brussels Airport. NATO’s supreme allied commander is based outside the city at SHAPE’s headquarters. It is currently Air Force Gen. Philip M. Breedlove, who is expected to retire this spring and be replaced by Army Gen. Curtis M. Scaparrotti.

NATO was established in 1949 following World War II after 12 nations signed the North Atlantic Treaty. Members agreed that “an armed attack against one or more of them in Europe or North America shall be considered an attack against them all.” The first supreme allied commander was Army Gen. Dwight D. Eisenhower, who established headquarters in Paris.

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NATO’s SHAPE was moved to Belgium in the 1960s, after France withdrew as a member of its integrated military structure, according to NATO. The supreme allied commander at the time, Army Gen. Lyman L. Lemnitzer, initially wanted SHAPE to be located near NATO’s headquarters in Brussels, but Belgian officials decided it must be at least 31 miles away from the city because it would be a target in a time of war.

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Belgium offered Camp Casteau, a military camp near Mons as a home for SHAPE, and agreed to build a highway connecting Mons and Brussels to ease NATO’s objections to not being centrally located in Brussels.