All flights and train services to and from Brussels have been cancelled following twin attacks, which killed 34 people, at the city's main international airport and Maelbeek Metro station on Tuesday morning.

According to public broadcaster VRT, a suicide bomber blew himself up at Brussels airport on Tuesday killing at least 14 people and a further blast tore through a rush-hour metro train in the capital shortly afterwards, claiming 20 lives.

A witness said he heard shouts in Arabic shortly before two blasts struck the packed airport departure lounge. Pictures on social media showed smoke rising from the terminal building through shattered windows and passengers fleeing down a slipway, some still hauling their bags.

Alphonse Youla, 40, who works at the airport, said 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 mangled," he said, his hands covered in blood.

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 had been on alert for any reprisal action.

Following the gruesome attacks, France has decided to deploy 1,600 additional police officers to bolster security at its borders and on public transport, French Interior Minister Bernard Cazeneuve said.

A total of 400 additional police officers will boost security in the greater Paris area, and military patrols will be refocused on public transport sites, the minister said after an emergency meeting with President Francois Hollande.

Meanwhile, German authorities have stepped up security measures at airports, train stations and the borders with Belgium, France, the Netherlands and Luxembourg following explosions in Brussels, a spokesman for the federal police said on Tuesday.

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. - Reuters