London's Gatwick airport stepped up security on Tuesday after a string of explosions in Brussels as British Prime Minister David Cameron prepared to hold an emergency cabinet meeting on the attacks.

A string of explosions rocked Brussels airport and a city metro station on Tuesday, killing at least 21 people, according to firefighters, as Belgium raised its terror threat to the maximum level.

"As a result of the terrible incidents in Brussels, we have increased our security presence and patrols around the airport," the airport said in a statement.

Cameron earlier said on Twitter he was "shocked and concerned" by theevents in Brussels.

"I will be chairing a COBRA meeting on the events in Brussels later this morning," Cameron said.

COBRA meetings are held to discuss how the government responds toemergency situations and bring together ministers, police and intelligence officers.

A string of explosions rocked Brussels airport and a city metro station on Tuesday, killing at least 21 people, according to firefighters, as Belgium raised its terror threat to the maximum level.

London's Heathrow, Europe's busiest airport, would not confirm it was stepping up security but said it was "working with the police at Heathrow who are providing a high visibility presence."

Eurostar trains to Brussels from London's St Pancras station have been suspended.