By KELLY STEVENSON, MEGHAN KENEALLY and LEE FERRAN

The terror group ISIS said today that its "fighters" were responsible for the terrorist attack in Belgium that claimed more than 30 lives and injured at least three Americans, according to an ISIS-affiliated website.

ISIS claimed it "carried out a series of bombings with explosive belts and devices" in "the Belgian capital Brussels, a country participating in the international coalition against the Islamic State [ISIS]," according to the site. The claim cannot be independently verified and there's no official word from Belgian authorities.

There were two explosions in the departure hall of Brussels Airport in Zaventem, Belgium, at about 8 a.m. local time today, according to Prime Minister Charles Michel, where officials said at least 14 people were killed. Three Americans, Mormon missionaries, were "seriously injured" in the blasts, according to Eric Hawkins, a spokesman for the Church of Jesus Christ of Latter-day Saints.

There was also an explosion a little over an hour later on the rail tracks at the Maelbeek subway station, according to a spokeswoman for Brussels transportation department. At least 20 people were killed there, officials said.

Between the two sites, local officials estimate that more than 180 people were injured.

President Obama addressed the tragedy while on a trip to Cuba, saying Americans "stand in solidarity" with Belgian people and that the United States will "do whatever is necessary to support our friend and ally Belgium in bringing to justice those responsible."

The Maelbeek station, where there was an explosion on the rail tracks, is located in central Brussels near European Union offices.

The subway system has been closed entirely and Brussels airport officials have been advising people not to travel to the airport.

A surveillance photo released hours after the attack showed three men that Belgian authorities identified as possible suspects in the airport bombing. It is unclear how the men were identified as potential suspects. ABC News does not yet have a confirmation on their identities or which, if any, are alive.

Belgian Ambassador to the U.S. Johan Verbeke told reporters that "statements ... cries" had been pronounced in Arabic at the airport, "so that is the first indication of what the source is maybe, but I don't want to elaborate on that because that is definitely something that has to be further investigated."

Verbeke said he did not know what was said in Arabic.

The explosions come as Salah Abdeslam, a suspect in the Paris attacks, was detained last week in Belgium by Belgian authorities.

The explosions have prompted an international response, with senior law enforcement officials in the United States confirming to ABC News that Homeland Security and FBI officials are monitoring closely. The New York Police Department has also confirmed that they are monitoring the situation closely.

President Obama has been briefed on the attacks, a White House official said.

A senior U.S. official told ABC News that U.S. Transportation Security Administration chief Peter Neffenger happened to be in Brussels and was actually on airport grounds at the time of the bombing there. He is in a safe place, the official said.

NATO, which is headquartered in Brussels, announced that it is increasing its "alert state," after decrying the "cowardly attack."

The French Interior Minister confirmed that 1,600 extra police officers have been deployed near French transportation hubs in the wake of today's explosions.

France had already sent out an extra 500 extra police near the country's borders after Abdeslam was arrested last week.

The French Army posted a message of support for Belgium on Twitter, along with a photo of soldiers with heavy weapons at what appears to be a transportation hub.

“More than ever, the French army is mobilized against the terrorist threat. We will win this war,” Gen. Pierre de Villiers said in a quote posted on the army's Twitter account.

The Metropolitan Police in London have released a statement confirming an increase in deployment, saying that "forces across the U.K. have increased policing presence at key locations, including transport hubs, to protect the public and provide reassurance. This is not in relation to any specific information or intelligence."

European Union officials in Brussels, the de facto capital for the E.U., have been told to stay in their offices or homes in the wake of the blasts.

So has the U.S. Embassy in Brussels, releasing a statement urging U.S. citizens to shelter in place and avoid all public transport.

ABC NEWS