Explosions have torn through the departure hall of Brussels airport, killing up to 13 people and injuring 35 others, while a second blast struck a metro station in the capital shortly afterwards, Belgian broadcaster VRT said.
One person has been confirmed dead by police, according the Belgium news agency Belga.
The blasts occurred around 8am (1800 AEDT) on Tuesday.
An explosion has also been reported at the Maelbeek metro station in Brussels close to EU institutions, RTBF says.
The Brussels transport authority says all metro stations in the city are closing.
An Australian embassy spokesman in Brussels told AAP staff were monitoring the situation and trying to determine if Australians were caught up in the explosions.
Belga news agency is reporting shots were fired and Arabic words were shouted before the explosions at the airport.
Airport authorities said the explosions caused several injuries with reports it was centred on an American Airlines desk in a departure hall.
The interior ministry says it's raising Belgium's security alert to the maximum Level 4 across the country.
The blasts occurred four days after the arrest in Brussels of a suspected participant in the November militant attacks in Paris that killed 130 people.
Belgian police had been on alert for any reprisal action.
The Belgian Prime Minister Charles Michel tweeted saying the government is monitoring the situation closely.
Airport spokeswoman Anke Fransen said: "There were two blasts in the departure hall. First aid team are in place for help."
Passengers were led on to the tarmac and the crisis centre urged people not to come to the airport.
Belgian media said rail traffic to the airport and flights out have been suspended.
Social media showed pictures of smoke rising from the departure hall where all windows had been shattered by the blasts. Passengers were seen running away down a slipway from the departure lounge.
#BREAKING: Bombs detonated next to American Airlines desk at Zaventem airport in Brussels: 10+ injured pic.twitter.com/ykDpsc6hgV — Amichai Stein (@AmichaiStein1) March 22, 2016
Sky News television's Alex Rossi, at the scene, 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."
"The thinking here is that it is some kind of terrorist attack - that hasn't been verified by any of the authorities here at the airport."
The incident comes after the arrest of Paris terror attack suspect Salah Abdeslam in the city last week.
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