At least six people were killed, while 11 others remain missing after a fire ripped through a hostel in Wellington, New Zealand on Tuesday.

The blaze broke out on the top floor of Loafers Lodge in the Wellington neighborhood of Newtown just after midnight, police said, causing major structural damage that is hampering recovery efforts.

"There is a significant amount of debris from the roof collapse but at this stage, we have located six people," Bruce Stubbs, the incident commander from Fire and Emergency New Zealand (FENZ) told a news conference.

The cause of the fire is not known, but emergency services were treating the incident as suspicious.

"This is a once-in-a-decade fire for Wellington. It’s the worst nightmare for us," FENZ Wellington District Commander Nick Pyatt said in a statement.

"This is a tragic event for all involved. My heartfelt condolences go to the loved ones of those who have lost their lives."

The hostel provided accommodation for people including construction workers, hospital staff and those serving sentences in the community for minor crimes.

Loafers Lodge resident Tala Sili said he saw smoke coming under his door and opened it to find the hallway full of smoke. He decided to jump out the window onto a roof two floors below.

"It was just scary, it was really scary, but I knew I had to jump out the window or just burn inside the building," he told state-owned Radio New Zealand.

Investigations into what caused the fire are underway, according to Fire and Emergency New Zealand (FENZ).

Police said they will not have more information until the 92-room building is safe to enter.

Dion Bennett, acting district commander of police for the area of Wellington where the fire broke out, said the force was working on the assumption that fewer than 10 people have died, but that others from the building were still unaccounted for.

"We have yet to fully reconcile a list of all of those people who were here last night who should be here or who were here as visitors," he said.

Prime Minister Chris Hipkins visited the site and spoke with emergency service providers.

"It is an absolute tragedy and it is a horrific situation," Hipkins told media after the visit.

"In the fullness of time, of course, there will be a number of investigations about what has happened and why it happened but for now, the focus needs to be on dealing with the situation."