Police presence at airports and other sensitive sites in Britain is being stepped up following the explosions in Brussels, Scotland Yard said Tuesday.

"As a precaution, forces across the UK have increased policing presence at key locations, including transport hubs, to protect the public and provide reassurance," said Assistant Commissioner Mark Rowley, in charge of counter-terrorism policing in Britain.

"This is not in relation to any specific information or intelligence."

He added that, in London, extra police were being mobilised to conduct "highly visible patrols at key locations."

British Prime Minister David Cameron is chairing an emergency meeting after three explosions at Brussels airport and a metro station close to EU institutions in the city killed at least 21 people.