Western Europe was on high alert on Tuesday after attackers launched twin assaults in Belgium’s capital Brussels with bombs ripping through the airport and the underground metro line. More than 30 people were killed and hundreds wounded – many in critical condition.

Belgian’s federal prosecutor told state media one of the explosions at Zaventem airport was “probably” a suicide attack.

Police were going door-to-door throughout Brussels searching for suspects or others planning attacks. The interior minister said 600 additional police were deployed.

“This is an attack against our values, our freedom, and our democracy,” Brussels Mayor Ivan Mayeur said at a press conference. “We won’t let anyone attack our values in a cowardly way.”

ISIL later claimed responsibility for the attacks.

“Islamic State fighters opened fire inside Zaventem Airport, before several of them detonated their explosive belts, as a martyrdom bomber detonated his explosive belt in the Maalbeek metro station,” reported the Amaq news agency, which is affiliated with ISIL.

The explosions struck the heart of European officialdom where NATO headquarters is based, along with the European Parliament and European Commission.

Footage from the airport – the country’s largest – showed people running from the terminal building as plumes of smoke rose to the sky. Fourteen people were killed and more than 90 wounded with the casualty toll steadily rising on Tuesday.

All metro lines were shut down after the attacks. Victims were seen fleeing with blood-splattered faces at the central Maelbeek station. At least 20 people were killed in the underground train blasts.

Ian McCafferty, a witness at the station, said the blast happened during busy rush hour traffic.

“People started running when they saw smoke coming,” he said. “The point of these attacks is to make you live in fear, but I refuse to.”

The interior ministry raised the country’s terrorism alert to the highest level after the blasts and Brussels’ crisis centre told people: “Stay where you are.”

Belgian Prime Minister Charles Michel called for unity as details were still emerging about the attacks.

“We are facing a difficult, challenging time and we should face up to this challenge by being united,” he said. “What we feared has happened, we were hit by blind attacks.”

Early reports from witnesses said they heard gunshots fired at the airport. Belgian’s federal prosecutor confirmed a Kalashnikov assault rifle was found there.

Abdeslam arrest

The country had been on high alert since the arrest in Brussels last week of Salah Abdeslam, a key suspect in last November’s Paris attacks that left 130 people dead.

Belgium’s Interior Minister Jan Jambon said on Monday the country was aware of the risk of a possible revenge attack following the capture of the 26-year-old.

Governments in Europe and beyond quickly responded to the Brussels attacks, calling emergency national security meetings and stepping up controls at airports and other sensitive sites.

Dutch Prime Minister Mark Rutte said his country would increase border patrols at the Dutch-Belgian borders and boost security.

France also announced increased security measures. President Francois Hollande said: “We are faced with a global threat ... The war against terrorism must be conducted across Europe.”

Nigeria’s President Muhammadu Buhari joined leaders across Europe and the rest of the world in condemning the atrocious terrorist attacks.

A statement issued in the Nigerian capital Abuja by Buhai’s Special Adviser to the President (Media & Publicity) Femi Adesina said: “President commiserates with Prime Minister Charles Michel and Belgians over the loss of over 30 lives in the attacks.

“The President assures Prime Minister Michel and the people of Belgium that having suffered the horror and anguish of incessant terrorist attacks over several years, Nigeria stands in full solidarity with them on this day of national pain and trauma.

“President Buhari believes that the appalling attack on Brussels reinforces the need for greater international cooperation to effectively confront and destroy global terrorism and its perpetrators.

“The President assures the global community that under his leadership, Nigeria will continue to work with other countries of the world to ensure that terrorism never triumphs over free, peaceful and law-abiding nations and people of the world.

“He wishes the more than 100 persons injured in the attacks a speedy recovery.”

•Partly based on an Al Jazeera report. Photo by Francois Lenoir/Reuters shows windows at Zaventem airport, Brussels, Belgium, shattered by Tuesday morning’s explosion.