Brussels' grand mosque has been evacuated after a suspicious envelope was found.

A spokesperson for the firemen deployed at the site, which is near the European Union headquarters, said the envelope contained a suspicious white powder, local media reported. The city's decontamination unit was also seen entering the mosque and 11 people were checked out for possible "contamination".

The city has been in lockdown last weekend as authorities believed that an Islamic State (Isis) cell was poised to launch terror attacks. Schools and the metro underground rail service reopened on 24 November.

Police in Belgium have arrested 21 people in counterterrorism operations since 22 November, but Paris attacks suspect Salah Abdeslam remains on the loose.

Abdeslam is suspected as one of three brothers involved in the massacre, although it is believed he abandoned carrying out his part of the suicide bombing. According to his brother Mohamed, Salah may have abandoned the device after deciding to "not pursue it to the end".

Meanwhile, Belgian police launched an anti-terrorism operation in Auvelais, 50km south of Brussels, linked with the possibility of attacks in the country.

A local police official in Sambreville told Reuters by telephone that an operation had been concluded but declined to say what the aim of it had been.

Highway E42 has been closed near the city, which sits between Charleroi and Namur.