US Secretary of State John Kerry's spokesman said the State Department has been "closely monitoring the situation".
"State Department officials are in touch with our embassy in Brussels. The embassy released a security message this morning, recommending that individuals shelter in place and avoid all public transportation. US citizens are urged to be aware of local events, follow local authority instructions and to monitor local media for further developments" he said.
"Our embassy will continue to update with further information when available. We continue to work to determine the status of all American citizens in Brussels. The United States stands with people of Belgium. We are ready to support the investigation as appropriate" the U.S. official added.
NATO Secretary General Jens Stoltenberg also called the attack "cowardly".
"My thoughts are with those who have lost their loved ones, with all those affected, and with the people of Belgium. We all stand together with our Ally Belgium on this dark day" he said.
"An attack on our values and on our open societies. Terrorism will not defeat democracy and take away our freedoms. We have decided to increase the alert state at NATO Headquarters. We remain vigilant and continue to monitor the situation very closely" Stoltenberg urged.
French President urges share of intelligience in "terror war"
French President Francois Hollande called on Europe to "share intelligence in terror war" speaking at a press conference.
He said that France and Belgium were "linked with horror" after Paris attacks in December. French government has taken measures to boost security at borders and at its ports, he said.
Hollande vowed France's full support to Belgian government, adding that "war against terrorists" should be enabled through the "whole of Europe using all the necessary means, especially sharing intelligence information".
Britain faces "a very real terror threat"
David Cameron has condemned terror attacks in Brussels as "appalling and savage", adding that Britain also faces "a very real terror threat". Security measures at ports, airports were bolstered he said.
"They could just as well be attacks in Britain or in France or Germany, or elsewhere in Europe and we need to stand together against these appalling terrorists and make sure they can never win" Cameron added.
Putin calls attacks "barbaric"
Russian President Vladimir Putin has condemned the attacks saying the acts were "barbaric", said his press secretary Dmitry Peskov.
"The president resolutely condemned these barbaric acts, expressed his condolences to the Belgian people and assured King Philippe of absolute solidarity with the Belgians in these difficult hours" Peskov said.
President of the European Parliament Martin Schultz has expressed condolences and condemned the attacks on a Twitter post.