One blast reportedly occurred at a subway station, and two more in the Brussels airport departure hall.
President Barack Obama, currently in Cuba on a foreign trip, said the U.S. "will do whatever is necessary to support" Belgium.
"This is yet another reminder that the world must unite. We must be together regardless of nationality or race or faith in fighting against the scourge of terrorism," he said at the top of a speech more broadly focused on Cuba.
U.S. Attorney General Loretta Lynch called the attacks "an appalling assault on the people of Belgium ... and every nation that stands for peace and the rule of law."
"The Department of Justice is in contact with our counterparts in Belgium and we have offered any and all assistance that we can bring to bear," she said in a statement. "In the days ahead, we will continue to work with law enforcement abroad in order to help ensure those responsible are brought to justice."
The U.S. was concerned about strike prior to attack, according to another U.S. official.
After the Brussels terror arrests this week, the official said concerns were obvious and the U.S. was worried other members of the terror cell could go underground, or regroup and strike.
Turkey had been the constant threat stream in recent weeks, the official said.
This is an example of the post al-Qaeda world, striking at the local level, in very infancy attacks, and grabbing the world stage, the official said.
The Department of Homeland Security tweeted, "DHS will not hesitate to adjust our security posture, as appropriate, to protect the American people. #Brussels."
NYC takes precautions
New York Gov. Andrew Cuomo has directed state law enforcement officials to "step up security at high-profile locations around the state, including our airports, bridges, tunnels and mass transit systems."
"Public safety is paramount, and I want the people of this state to know that we are working with all local and federal partners, remaining vigilant and taking all necessary measures to keep New Yorkers safe," he said in a statement.
New York Police Department spokesman J. Peter Donald said despite the increased efforts "at this time, there is no known indication that the attack has any nexus to New York City."
"The department has deployed additional counterterrorism resources across the city including: the Counterterrorism Response Command, the Strategic Response Group and Hercules Teams," he said. "These teams have been deployed to crowded areas and transit locations around the city out of an abundance of caution to provide police presence and public reassurance as we closely follow the developing situation overseas."
The stepped up security also applies to the Long Island Rail Road transit system at Penn Station, Metro North at Grand Central Station and other major hubs like the Atlantic Terminal in Brooklyn, according to a Metropolitan Transit Authority senior law enforcement official.
The Port Authority Police Department is stepping up security at its three major airports, both LaGuardia Airport, John F. Kennedy International Airport in New York, and Newark Liberty International Airport in New Jersey, according to Joe Pentangelo, a Port Authority spokesman.
The Port Authority Police Department is also employing high visibility anti-terrorist patrols at Port Authority Trans Hudson line and World Trade Center, Pentangelo said.
Port Authority will have a presence at both the World Trade Center transit hub and the World Trade Center at large, according to Scott Ladd, spokesman for the Port Authority of New York and New Jersey.
Other U.S. cities react
Airports in cities across the country -- including Atlanta, Boston, Chicago, Denver, Los Angeles, Miami, and Philadelphia -- stepped up security in the wake of the attacks, though there are no known threats, according to officials.
In Washington, police across the Metropolitan Transportation Authority metro system will increase K9 sweeps and police patrols in the wake of terror attacks in Belgium, according to the agency's twitter feed.
There are also plans are underway to roll out a major show of police force at Reagan National Airport and Dulles International Airport, in the Washington area, which will include SWAT, bomb sniffing K9's, and a general increase police presence, according to an aviation official.
At Union Station in Washington, police are pulling random passengers as they are boarding to go through luggage security checks. Not everyone is going through luggage checks, but bomb sniffing dogs are also out on the tracks doing sweeps.
Amtrak said in a statement it is working with state, local and federal officials and "extra officers have been deployed."
The Pentagon has also increased security, according to a spokesperson.
In Boston, a Massport official declined to elaborate on any security changes at Logan International Airport.
"Our airport security program is multilayered and constantly evolving," Massport spokesman Matt Brelis told CNN. "We do not discuss specific tactics—including those that may rely on technology."
Atlanta Hartsfield-Jackson International Airport is now operating on heightened security alert, according to spokesman Reese McCranie.
A spokeswoman for Los Angeles International Airport says at this time there are "no specific threats" but as a precaution they have increased the visibility of airport police.
Republican front-runner Donald Trump tweeted Tuesday that the U.S. must be vigilant and smart following the explosion.
"Do you all remember how beautiful and safe a place Brussels wss. Not anymore, it is from a different world! U.S. must be vigilant and smart," he tweeted.
During an interview on Fox News, Trump said Brussels has transformed
from a place of tranquility to an "armed camp."
"Twenty years ago, like a magical city, now you look at it its an armed camp, looking at pictures on TV now it's horrible," he said. "You want to lead your life, not live in armed camp, and certain group is making living a normal life impossible."
Ohio Gov. John Kasich said he was "sickened by the pictures of carnage."
"The wave of terror that has been unleashed in Europe and elsewhere around the world are attacks against our very way of life and against the democratic values upon which our political systems have been built," he said in a statement. "We and our allies must rededicate ourselves to these values of freedom and human rights. We must utterly reject the use of deadly acts of terror."
Texas Sen. Ted Cruz called the blasts "just the latest in a string of coordinated attacks by radical Islamic terrorists perpetrated those who are waging war against all who do not accept their extreme strain of Islam," in a Facebook post.
"Radical Islam is at war with us," he said, and accused Obama of refusing to "acknowledge this reality."
Clinton, Sanders respond
Democratic front-runner Hillary Clinton told NBC's "Today" show the U.S. has to be "absolutely smart and strong and steady in how we respond."
"We've got to stand in solidarity with our European allies," she said.
But the former secretary of state cautioned against blanket bans on immigrants based on the attacks.
"It's unrealistic to say we're going to completely shut down our borders to everyone," she said. "I know that Americans have every reason to be frightened by what they see, (but) we've got to work this through, consistent with our values."
Clinton said in a statement that "the people of Brussels, of Europe and of the world will not be intimidated by these vicious killers."
"Today's attacks will only strengthen our resolve to stand together as allies and defeat terrorism and radical jihadism around the world," she said.
Vermont Sen. Bernie Sanders called the attack "another cowardly attempt to terrorize innocent civilians."
"Today's attack is a brutal reminder that the international community must come together to destroy ISIS. This type of barbarism cannot be allowed to continue," he said in a statement.
He also tweeted, "We offer our deepest condolences to the people of Brussels and stand with our European allies to offer any necessary assistance."