BRUSSELS (AFP) - Belgian police surrounded an apartment in the city of Ghent on Monday after four armed men were believed to have taken a hostage, but there were no immediate indications of terrorism, prosecutors said.

Special armed police set up a wide perimeter around the building after the siege started several hours ago in a working-class area of the north-western, Flemish-speaking city.

"There is no indication that this is linked to terrorism, the Islamic State or what is happening at the moment in Sydney," the prosecutor said.

The Belgian siege happened at the same time as a lone gunman kept terrified staff and customers captive in a cafe in Australia's largest city, brandishing an Islamic flag.

Ghent's local prosecutor said that "four armed men" were involved and that one hostage was believed to have been taken but stressed that it appeared to be an isolated incident with no terror links.

A person in the building, which is divided into several flats, saw the armed men and was able to leave before calling the police to alert them, a police spokesman said.