Brussels is "a disaster city" where assimilation has failed, U.S. Republican presidential frontrunner Donald Trump said Tuesday after a series of apparently coordinated explosions rocked the Belgian capital's airport and a metro station.

At least 26 people were killed in the latest attacks to target Europe, and Trump wasted little time in saying it was more evidence that governments needed to crack down on extremists with any means possible -- even using waterboarding -- and that immigration policies had failed.

Pope Francis on Tuesday slammed the "blind violence" of a series of coordinated explosions in Brussels that killed at least 26 people, offering his prayers for the victims.

"Pope Francis entrusts to God's mercy those killed, and shares through prayer the suffering of their relatives," the Vatican's Secretary of State Pietro Parolin said in a telegram to Jozef De Kesel, the archbishop of Mechelen-Brussels.

New York and Washington D.C. stepped up security in the wake of the attacks in Brussels on Tuesday, deploying elite counter-terrorism reinforcements to crowded areas and train stations, police said.

The New York police department said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute.

Hizbullah condemned on Tuesday the bombings that shook the Belgian capital Brussels, saying they were committed by the “takfiri terrorist groups” that are not sparing anyone in the world.

It said in a statement: “The fire that is blazing in the world, Europe in particular, is the same one that was ignited by some powers against Syria and other countries in the region.”

European countries on Tuesday vowed to defend democracy against terrorism after blasts at Brussels airport and in the EU's institutional heart left around 35 dead and more than 200 injured.

As Belgium's allies voiced anger and solidarity, Russia and Turkey -- themselves targets of deadly attacks in the last eight months -- said the blasts rammed home the need to fight terrorism of every hue.

Sunni Islam's leading seat of learning, Al-Azhar, said Tuesday's attacks in Brussels "violate the tolerant teachings of Islam," and urged the international community to confront the "epidemic" of terrorism.

"Al-Azhar strongly condemns these terrorist attacks. These heinous crimes violate the tolerant teachings of Islam," the Cairo-based Al-Azhar said in a statement.

Airlines canceled hundreds of flights and European railways froze links with Brussels Tuesday after a series of bombs blasts killed around 35 people in the city's airport and a metro train, sparking a broad security response.

Belgium locked down the capital, home to the headquarters of the European Union and NATO, and imposed its highest level of security alert after the explosions, which extinguished about 20 lives in the metro and another 14 in the airport, according to authorities in Brussels.

Victims lay in pools of blood, their limbs blown off, as the smoke cleared to reveal a scene of horror after twin explosions ripped through the main terminal at Brussels airport, witnesses told AFP.

The normally bustling departure hall at Zaventem was wrecked by the morning rush-hour blasts, with part of the ceiling collapsing near the check-in desks and many of the huge plate glass windows blown out by the attack.

British Prime Minister David Cameron condemned the "appalling" attacks in Brussels and said Europe should stand together against the terror threat, following an emergency cabinet meeting in London on Tuesday.

"We will never let these terrorists win," Cameron said at his Downing Street office.

Police have found the DNA of a newly-identified suspect on explosives used in last year's Paris attacks, a French source revealed Monday, as Belgian and French prosecutors met in Brussels to discuss the probe into the November carnage.

The suspected accomplice was named as Najim Laachraoui, who was previously known by the false name Soufiane Kayal which he used to travel to Hungary in September with Salah Abdeslam, the key suspect who was arrested in a dramatic raid on Friday.