NEW YORK: Major US transportation hubs were placed on alert on Tuesday and Denver International Airport briefly evacuated part of its main terminal in a false alarm there hours after suicide bombings in Brussels killed at least 30 people.

Despite public safety concerns unleashed by the violence in Belgium's capital, US Homeland Security Secretary Jeh Johnson said the agency had no intelligence that would point to a similar attack being plotted against the United States.

But the State Department issued a travel alert warning US citizens in Europe to avoid crowded places, to be vigilant when in public or using mass transit and to exercise extra caution during religious holidays and at large festivals and events.

"Terrorist groups continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants, and transportation," it said in a statement.

The Brussels bombings reverberated on the 2016 US presidential campaign trail, with Democratic contender Hillary Clinton declaring that more needed to be done to confront the Islamic State militants who claimed responsibility for the attacks.

The Republican front-runner in the White House race, Donald Trump, called again for tighter border security and suggested US intelligence services could use torture to head off future attacks.

Some of the country's busiest airports and other transportation facilities were placed on heightened security status, as illustrated by a greater law enforcement presence.

Large numbers of uniformed police officers and National Guard troops dressed in battle fatigues and carrying rifles patrolled New York's John F. Kennedy International Airport. Several U.S. carriers - Delta Air Lines Inc (DAL.N), United Continental Holdings Inc (UAL.N) and American Airlines Group Inc (AAL.O) - said they cancelled or rerouted flights as a result of the Brussels attacks.

At mid afternoon, authorities at the Denver airport evacuated two levels on the west side of the main terminal after several packages that appeared suspicious were spotted near ticket counters, airport spokeswoman Stacey Stegman said.

Denver police, FBI and US Transportation Security Administration officers converged on the airport, but the packages were ultimately deemed to pose no threat, and the terminal was fully reopened within two hours.

Several airlines were affected by the scare, including American Airlines, Aeroméxico, Air Canada, Lufthansa and British Airways, the airport said.

'WORLD MUST UNITE'

US President Barack Obama ordered flags flown at half-staff in memory of the victims in the Belgium attacks.

The State Department said an undetermined number of US citizens had been injured in Brussels but none were killed. Three Mormon missionaries and a US Air Force member and his family were among those hurt.

The Obama administration also was expected to impose tighter security measures at US airports following the Brussels Airport bombings, which occurred in a public hall outside of the security check area.

US Representative William Keating of Massachusetts, senior Democrat on a House subcommittee on terrorism, said the suicide bombings illustrated the difficulty of protecting "soft targets" outside tightly controlled security cordons.

"The targets aren't going to be just getting on the plane itself, but the airport in general," he said in a phone interview.

Obama addressed the attacks briefly in a speech in Havana on his historic visit to Cuba, vowing to support Belgium as it hunts for those responsible.

"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," Obama said.

Candidates seeking their party's nomination for the Nov. 8 presidential election immediately weighed in, with Clinton, a former secretary of state, vowing to strengthen her drive to "defeat terrorism and radical jihadism."

Trump, a billionaire businessman, told NBC's "Today" program: "If they could expand the laws, I would do a lot more than waterboarding. You have to get the information from these people."

His Republican rival, US Senator Ted Cruz of Texas, strengthened his call for Obama to clip the flow of refugees from “countries with significant al Qaida or ISIS presence,” and called for heightened police scrutiny of neighbourhoods with large Muslim populations.

The attack raised worries among some US Muslims that they could face more hostility, although mainstream Muslims have repeatedly denounced violence.

"The media hype and political manipulation heightens our concerns," said Sheikh Shaker Elsayed, imam of the Dar Al-Hijrah mosque in Virginia.

Some travellers expressed concern that new security measures at airports, which had already imposed extensive restrictions since the Sept. 11, 2001, attacks on New York and Washington, would increase inconvenience without improving safety.

"It already takes all day," said Hans Vermulst, 66, who was at New York's Kennedy airport trying to get home to the Netherlands after his connecting flight to Brussels was cancelled. "We have to take it as it comes, but I'm not happy with it."