At least 26 people were killed in twin attacks on Brussels airport and a rush-hour metro train in the Belgian capital on Tuesday, triggering security alerts across western Europe and bringing some cross-border transport to a halt.

A witness said he heard shouts in Arabic shortly before two blasts struck a packed airport departure lounge at Brussels airport. The federal prosecutor said one of the blasts was probably triggered by a suicide bomber.

The Belgian health minister said 11 people were killed in the airport bombing and 81 wounded.

The blasts at the airport and metro station occurred four days after the arrest in Brussels of a suspected participant in November militant attacks in Paris that killed 130 people.

Belgian police and combat troops on the streets had been on alert for any reprisal action but the attacks took place in crowded public areas where people and bags are not searched.

Video showed devastation in the hall with ceiling tiles and glass scattered across the floor. Some passengers emerged from the terminal with blood spattered over their clothes. Smoke rose from the building through shattered windows and passengers fled down a slipway, some still hauling their bags.

Many of the dead and wounded were badly injured in the legs, one airport told Reuters, suggesting at least one bomb in a bag.

Britain, Germany, France and the Netherlands, all wary of spillover from conflict in Syria, were among states announcing extra security measures.

All public transport in Brussels was shut down, as it was in London during 2005 islamist militant attacks there that killed 52. Authorities appealed to citizens not to use overloaded telephone networks, extra troops were sent into the city and the Belgian Crisis Centre, clearly wary of a further incident, appealed to the population: "Stay where you are".

British Sky News television's Alex Rossi, at the airport, said he heard two "very, very loud explosions".

"I could feel the building move. There was also dust and smoke as well...I went towards where the explosion came from and there were people coming out looking very dazed and shocked."

Alphonse Youla, 40, who works at the airport, told Reuters he heard a man shouting out in Arabic before the first explosion. "Then the glass ceiling of the airport collapsed."

"I helped carry out five people dead, their legs destroyed," he said, his hands covered in blood.

A witness said the blasts occurred at a check-in desk.

Belgian Prime Minister Charles Michel spoke of "a black time for our country".

"What we feared has come to pass. Our country has been struck by attacks which are blind, violent and cowardly."

The STIB public transport operator said 15 were killed on board the metro train and 55 injured. Belga news agency cited the fire brigade as saying 11 were killed at the airport, but there was still some uncertainty about casualties.

The blast hit the train as it left Maelbeek station, close to European Union institutions, heading to the city centre.

The VRT public broadcaster carried a photograph of a metro carriage at a platform with doors and windows completely blown out, its structure deformed and the interior mangled and charred.

A local journalist tweeted a photograph of a person lying covered in blood among smoke outside Maelbeek metro station, on the main Rue de la Loi avenue which connects central Brussels with the EU institutions. Ambulances were ferrying the wounded away and sirens rang out across the area.

The federal prosecutor told a news conference one of the two explosions at the airport was likely to have been caused by a suicide bomber.

FLIGHTS CANCELLED, PASSENGERS EVACUATED

Belgian Prime Minister Charles Michel told the news conference: "What we had feared has come to pass. Our country has been struck by attacks that are blind, violent and cowardly."

British Prime Minister David Cameron, whose country is also on a high security alert, expressed shock over the attack. "We will do everything we can to help."

Brussels airport said it had cancelled all flights until at least 6 a.m. (0500 GMT) on Wednesday and the complex had been evacuated and trains to the airport had been stopped. Passengers were taken to coaches from the terminal that would remove them to a secure area.

All three main long-distance rail stations in Brussels were closed and train services on the cross-channel tunnel from London to Brussels were suspended.

Security services have been on a high state of alert across western Europe for fear of militant attacks backed by Islamic State, which claimed responsibility for the Paris attack.

While most European airports are known for stringent screening procedures of passengers and their baggage, that typically takes place only once passengers have checked in and are heading to the departure gates.

Although there may be discreet surveillance, there is nothing to prevent member of the public walking in to the departure hall at Zaventem airport with heavy baggage.

Following an attempted ramraid attack at Glasgow Airport in 2007, several airports stepped up security at entrances by altering the pick-up and drop-off zones to prevent private cars getting too close to terminal buildings.

European stocks fell after the explosions, particularly travel sector stocks including airlines and hotels, pulling the broader indices down from multi-week highs. Safe-haven assets, gold and government bonds rose in price.

French citizen Salah Abdeslam, the prime surviving suspect for November's Paris attacks on a stadium, cafes and a concert hall, was captured by Belgian police after a shootout on Friday.

Belgium's Interior Minister, Jan Jambon, said on Monday the country was on high alert for a revenge attack.

"We know that stopping one cell can ... push others into action. We are aware of it in this case," he told public radio. – Reuters