2 explosions at Brussels airport, 1 at subway station; reports of several killed

Brussels, Belgium (CNews)[Breaking news update, posted at 6:01 a.m. ET]

At least 23 people have been killed in three explosions in Brussels, Belgium, according to Belgian media.

Ten were killed at a subway station, according to two Belgian broadcasters, and at least 13 were killed at the airport, according to public broadcaster VRT.

[Breaking news update, posted at 5:50 a.m. ET]

The terror threat level in Belgium has been raised to four, its highest level. There were three explosions in Brussels on Tuesday -- one at a subway station and two at the airport. Raising the threat level to four means authorities can deploy the army onto the streets to meet security needs.

[Previous story, posted at 5:47 a.m. ET]

(CNews) -- A series of blasts throughout Brussels on Tuesday have killed multiple people.

Two explosions occurred at the city's airport. Several people were killed, according to the country's public broadcaster, RTBF. Initial reports say up to 10 people were killed and more than 30 people were injured.

Another blast at a subway station occurred in the suburb of Maalbeek, near the European quarter, where much of the European Union is based, according to CNews affiliate RTL. All metro stations in the city have been closed, according to the official Twitter account for the city's transportation agency.

It's unclear what caused the blasts, or if they're related to terrorism.

The airport

Anthony Barrett said he heard something at about 8 a.m. local time from his hotel across from the terminal building.

"When I opened the curtains and looked out, I could see people fleeing," he told CNews.

He said he's seen about 19 or 20 stretchers carrying people so far. Luggage trollies were also being used to transport the wounded.

"It's clearly a very serious incident," he said.

Federal police at the airport at Zaventem told CNews that "there has been an explosion" and "something has happened."

Witnesses told RTL at least one of the two explosions took place in the airport's departure hall.

Dozens of people have been taken on stretchers out of the airport, according to eyewitnesses.

The Brussels airport tweeted that there have been two blasts and said that "the building" is being evacuated. They also said all airport operations have been suspended and asked those nearby to remain calm.

The airport is currently being evacuated and a disaster plan has been initiated, according to CNews affiliate RTL. Passengers have been directed to the runways.

One of them, Jeffrey Edison, had cleared security and was out by the gate several hundred yards away from the departure lounge, where the explosions occurred. He told CNews he didn't hear the blasts but "suddenly saw" 200 to 300 people rushing toward him from the security checkpoint.

He says it took authorities around 25 minutes to inform the passengers of what had happened, before evacuating the area and leading the passengers to the airport's runways.

The metro

The Belgian crisis center tweeted that all public transportation in the city has been closed.

"Stay where you are," it said.

Kristalina Georgieva, the vice president of the EU Commission, said that all the organization's institutions are at "alert level orange" and that all meetings on its premises and outside have been canceled.

She advised people to stay at home or indoors.

"I am shocked and concerned by the events in Brussels," British Prime Minister David Cameron tweeted. "We will do everything we can to help."

Eurostar said its trains are not running to or from Brussels' Midi station.