The attacks 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. New York Police Department officials have also confirmed that they are monitoring the situation closely.

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.\"

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

Get real-time updates as this story unfolds. To start, just \"star\" this story in ABC News' phone app. Download ABC News for iPhone here or ABC News for Android here.

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