Four armed men have entered an apartment in the Belgian city of Ghent and authorities have closed the surrounding area.

Unconfirmed reports indicate that the four men have taken one male hostage after they stormed an apartment in the northern city's Dampoort area but it is unknown whether the situation is related to the ongoing Sydney cafe hostage siege.

"They're in the building but it's not clear what's going on inside," a police spokesman said.

Belgian authorities have ordered residents in the city to remain indoors for safety reasons.

The state prosecution service said that there was no immediate indication that the incident was linked to terrorism and there was no further information about the four armed men.

Ghent police chief Filip Rasschaert told local outlet De Standaard that authorities were conducting their operation with caution because of the possibility of a hostage inside the apartment.

"They have all they need to bring a successful end to a hostage-taking," he said.

"There is information that a hostage might have been taken, so we err on the side of caution."

An armed gunman is holding approximately 15 people hostage in a Lindt cafe in Sydney's Martin Place, forcing a number of them to hold up a black Islamic flag against the window.