PENTAGON / WASHINGTON - U.S. officials have renewed calls to defeat Islamic State and are taking immediate protective steps against new terrorist attacks, after Islamic State militants claimed responsibility for three deadly explosions in Brussels.
U.S. Secretary of Defense Ash Carter said the U.S. and its allies will "do everything we can to protect our homelands" from terrorism.
"No attack will affect our resolve to accelerate the defeat of ISIL," Carter said Tuesday on Capitol Hill, using an acronym for the Islamic State terror group.
After explosions ripped through the airport and a subway station near the European Union headquarters, the U.S. embassy in Brussels has recommended Americans stay where they are and avoid public transportation in the Belgian capital. It said U.S. citizens there should monitor media reports, follow instructions from authorities and "take the appropriate steps to bolster your personal security."
Homeland Security, Intelligence officials monitoring
The U.S. Homeland Security agency said it is closely monitoring developments in the Brussels attacks and "will not hesitate to adjust our security posture, as appropriate, to protect the American people." It urged the public to immediately report any suspicious activity to local authorities wherever they are.
One senior U.S. intelligence official said, "The intelligence community continues to assess the situation in Brussels and is staying in close contact with our Belgian and European partners."
Speaking in a House Armed Services committee hearing Tuesday with Secretary Carter, General Joe Dunford, the Chairman of the Joint Chiefs of Staff, called for more intelligence sharing among international allies.
"There's a lot of walls to break down in order for us to be effective," said Dunford, adding that more than 100 nations have foreign fighters who have joined up with Islamist extremists in Iraq and Syria.
President Barack Obama, on the third day of a visit to Cuba, was briefed on the Brussels attacks.
New York, Washington boost security
New York, Washington and Chicago authorities boosted security patrols in the wake of the attacks.
New York police officials said there was no indication the Brussels explosions were related to the biggest U.S. city, but noted officers had been dispatched to crowded areas and transit locations "out of an abundance of caution to provide police presence and public reassurance."
The city's port authority also increased anti-terrorist patrols at the city's three airports, bridges, tunnels and bus terminal.
In Washington, the regional subway system sent officers with bomb-sniffing dogs into train stations for security checks, although it too said the patrols were precautionary and that there were no known credible threats against the national capital.
In the large Midwestern city of Chicago, extra police and canine units were dispatched to airports and transit stations.
Chicago Mayor Rahm Emanuel said, "While an ocean may separate us, Chicago and Brussels are united by common values of life, liberty and the pursuit of happiness."
National Security correspondent Jeff Seldin contributed to this report