US President Barack Obama has condemned the “outrageous attacks” in Brussels, which killed dozens of people and injured more than 200 others, including three Mormon missionaries.
Speaking on the final day of his trip to Cuba, Mr Obama began his speech by saying: “We stand in solidarity with them in condemning these outrageous attacks against innocent people.
“We will do whatever is necessary to support our friend and ally Belgium in bringing to justice those who are responsible,” he added.
He said Tuesday’s explosions at Brussels airport and subway system were another reminder the world must unite in “fighting against the scourge of terrorism”.
The Church of Jesus Christ of Latter-day Saints said three of its missionaries serving in Paris were seriously injured in the blasts.
It named them as 66-year-old Richard Norby, 20-year-old Joseph Empey and 19-year-old Mason Wells.
Secretary of State John Kerry, speaking from Cuba, said the US was trying “to determine the status of all American citizens in Brussels”.
The Brussels embassy warned Americans to stay where they are and “take the appropriate steps to bolster your personal security”.
Congressman Adam Schiff, ranking member of the House intelligence committee, said the explosions “bear all the hallmarks” of an Islamic State group attack.
The US Department of Homeland Security said it “would not hesitate to adjust our security posture, as appropriate, to protect the American people”.
The Justice Department said the FBI was co-ordinating with other US government agencies and its Belgian counterparts.
Also reacting to the attacks, Republican presidential front runner Donald Trump said Brussels was “a disaster city” where assimilation and immigration policies had failed.
He said the blasts underscored the need to crack down on extremists by any means, even waterboarding.
Mr Trump, who has called for a temporary ban on Muslims entering the US, reiterated his belief the US should close its borders “until we figure out what’s going on”.
He told NBC: “Belgium is no longer Belgium. Belgium is not the Belgium you and I knew from 20 years ago, which was one of the most beautiful and safest cities in the world.
“Belgium is a horror show right now. Terrible things are happening. People are leaving. People are afraid. This all happened because, frankly, there’s no assimilation.”
Democratic presidential front runner Hillary Clinton said the attacks will “only strengthen our resolve to stand together as allies and defeat terrorism and radical jihadism”.