People leave the scene of explosions at the airport. Twenty more were killed in a subway train blast during rush hour. [Photo/Agencies]

Officials at China's embassy in Belgium said that there had been no reports so far of Chinese killed or injured in the attacks. The emergency unit of University Hospital Saint-Luc, which is near the airport, said there were no Asians known to be among those sent there for treatment.

The embassy has asked Chinese to exercise caution if they plan to visit Belgium.

Belgian public broadcaster VRT put the death toll from the attacks in Brussels at 34, with 20 people killed in the blast on a subway train and 14 in explosions at the airport.

"What we feared has happened," Belgian Prime Minister Charles Michel told reporters. "In this time of tragedy, this black moment for our country, I appeal to everyone to remain calm but also to show solidarity."

Michel canceled his visit to China. He had been scheduled to attend the Boao Forum this week.

"The visit has been canceled due tow hat happened in Brussels," a source told China Daily on Tuesday.

A witness said he heard shouts in Arabic shortly before two explosions struck a packed departure lounge at Brussels Airport. A federal prosecutor said one of the blasts was probably triggered by a suicide bomber.

There was still some uncertainty about the number of casualties.

The blasts at the airport and subway station occurred four days after the arrest in Brussels of Salah Abdeslam, a key suspect in the terror attacks in Paris that killed 130 people in November.

Belgium's security alert was raised to the highest level after the attacks. Police and troops on the streets were on alert for any further attacks.