Belgian authorities have charged a man with involvement in "terrorist attacks" as investigations continue into the massacre in Paris two weeks ago.

The federal prosecutor's office said the unidentified man was "charged with terrorist attacks and taking part in the activities of a terrorist group".

He was arrested in Brussels on Thursday.

The prosecutor's office also announced that a man who drove fugitive Salah Abdeslam through the Belgian capital after the attacks will be kept in custody for at least a month.

A court has confirmed the continued detention of the man, identified in official documents as Ali O. He has not been charged.

His lawyer Olivier Martins told Belgian media that Abdeslam admitted to Ali O. that his brother Brahim blew himself up and killed people in Paris.

Mr Martins insisted that his client is innocent, saying "you're not going to tell me that if you drive Salah Abdeslam a few kilometres through Brussels that you're part of a terrorist group".

The city's schools and subway system reopened on Wednesday but the alert level is expected to remain high - on level 4 out of 5 - across the country this week.

Meanwhile, German prosecutors are investigating whether a man they have in custody may have sold four of the assault rifles used in the attacks.

The 24-year-old German man is suspected of delivering four Kalashnikov rifles to a Paris address, prosecutors said, adding that it is not clear who the man sold them to.

The unidentified man is also accused of converting legal starter pistols to fire live ammunition and selling them on the Internet.

A spokesman said it was in connection with investigating that operation that authorities learned of the possible Paris connection.