Victims lay in pools of blood, their limbs blown off, as the smoke cleared to reveal a scene of horror after twin explosions ripped through the main terminal at Brussels airport, witnesses told AFP.

The normally bustling departure hall at Zaventem was wrecked by the morning rush-hour blasts, with part of the ceiling collapsing near the check-in desks and many of the huge plate glass windows blown out by the attack.

"A man shouted a few words in Arabic and then I heard a huge blast," airport baggage security officer Alphonse Lyoura, who still had blood on his hands following the explosion, told AFP.

He said there was another explosion about two minutes later.

"I helped at least six or seven wounded people. We took out some bodies that were not moving. It was total panic everywhere," Lyoura said.

"I saw people lying on the ground covered in blood who were not moving.

"At least six or seven people's legs were totally crushed. A lot of people lost limbs. One man had lost both legs and there was a policeman with a totally mangled leg."

Emergency services said at least 11 people were killed in the blast and many others wounded.

The city was already on high alert after the arrest of Salah Abdeslam, a key suspect in the Paris terror attacks in November that were claimed by the Islamic State group.

'A TERRIBLE MESS'

Jean-Pierre Lebeau, a French passenger who had just arrived from Geneva, told AFP: "We heard the explosion and felt the blast."

He said he had seen many wounded and "blood in the elevator."

"First we were kept together by the police, then they gave us the order to evacuate," he said, recounting the shock on people's faces and a smell seemingly of gunpowder at the scene.

Michel Mpoy, 65, who was at the airport to pick up a friend coming from Kinshasa, said it was "a total mess -- it was terrible".

An employee for the Swissport airport management company described how she looked after a child following the blasts.

"I heard the first explosion and I took a child in my arms and hid him under the counter. Then I gave him to a policeman," the employee said without giving her name.

"There were injured people lying everywhere and some weren't moving."

At the airport, on the northwest outskirts of the city and not far from Nato headquarters, Jean-Pierre Herman said he was relieved to have got out safely.

Herman embraced his wife Tankrat Paui Tran, who he had just gone to collect from the airport after her flight from Thailand.

"My wife just arrived," Herman said. "I said 'hello,' we took the elevator and in the elevator we heard the first bomb.

"The second exploded just when we got off. We ran away to an emergency exit. I think we are very lucky."

Charlotte McDonald-Gibson, a British journalist living in Brussels, told AFP there had been "total confusion" at the airport, where she was having breakfast before a flight.

"Suddenly staff rushed in and said we have to leave," she said. "They rushed out and into the main terminal A departures building. Nobody knew what was going on."

"It was total confusion, people were just standing around wondering what was happening."

Another blast about half an hour after those at the airport hit the Brussels metro between the Maalbeek and Schuman stations in the European Union quarter of Brussels, which is also home to major international organisations and companies.

Around 10 people were killed, emergency services said.

SIRENS WAILING

AFP journalist Lachlan Carmichael was on the metro and described how his train was halted in the tunnel and then evacuated as it began to fill up with smoke.

A police officer told him: "There are wounded, there are dead, I do not know how many."

The officer was escorting a woman through a police cordon being put up around the Maalbeek station, with all public transport being closed down.

Another AFP journalist, Cedric Simon, said the situation around the Maalbeek metro station was totally confused, with a cloud of smoke and dust settling over the road outside.

Simon said there were about 15 people lying on the roadside, many with bloodied faces and being treated by medical staff as all Brussels hospitals were put on standby to deal with casualties.

The streets were filled with police cars and emergency vehicles, sirens wailing and blue lights flashing.

Brussels has been on high alert since the attacks in Paris in January, with heavily armed police and then troops put on the streets.

Troops were clearly very watchful as they patrolled outside EU and other institutions in the Maalbeek-Schuman area.