People in Brussels are being urged to avoid busy areas such as concerts and transport hubs after the city's terror alert was raised to the highest possible level.

The Belgian capital's metro transport service will be closed until Sunday afternoon over the "serious and imminent threat".

Belgium's prime minister said the terror alert was as a result of "quite precise information about the risk of an attack like the one that happened in Paris".

Heavily armed soldiers and police are patrolling the city amid fears at least one suspect in the Paris attacks could be in Belgium.

Belgium's crisis centre website said it had asked local authorities to cancel large events, urge people to avoid crowds and postpone soccer matches.

Belgian PM Charles Michel said the government would review the security situation on Sunday afternoon.

It comes after a meeting of top ministers, police and security services in the city, which is also home to the European Union and NATO headquarters.

A spokesman for the Belgium government's crisis centre said in a statement: "Following our latest evaluation... the centre has raised its terror alert to level 4, signifying a very serious threat, for the Brussels region.

"The analysis shows a serious and imminent threat requiring specific security measures as well as detailed recommendations to the population."

The centre urged citizens to avoid crowds in the capital, which is also home to the European Union and NATO headquarters.

Belgium, and Brussels in particular, have been at the centre of investigations into the Paris attacks after it emerged that two of the suicide bombers had been living in the country.

Three people detained in Brussels are facing terrorism charges.

The brother of one of the suicide bombers, who was also living in Brussels, is still on the run.

Salah Abdeslam was briefly pulled over by French police near the Belgian border last Saturday morning along with two of those in custody.

Some Belgian media speculated that the heightened state of alert related to the continued search for him.

However, the crisis centre spokesman declined to say what had led to the status change because investigations were proceeding.

"We cannot give more information... The work of federal prosecutors is still going on," he said, adding the government was assessing what extra security measures to take.

Soldiers are already on guard in certain parts of Brussels.

The last time any part of the country was put on maximum alert was in May 2014 when a gunman shot dead four people at the Jewish Museum in Brussels.

At that time, Jewish schools, synagogues and other institutions were put on level four.