Peter Whittall, the CEO of Pike River Coal, said 'my love and support are with those guys' [EPA]

Rescue teams in New Zealand have begun preparing to recover bodies of 29 miners believed to have died after they were trapped underground for days in a coalmine following an explosion.

Officials said on Thursday it could take weeks or even months to recover the bodies as New Zealand held a national day of mourning for the victims of the country's worst mining disaster in nearly 70 years.

Recovery teams were prevented from entering the mine by high levels of potentially explosive methane and other toxic gases that are blamed for two huge blasts inside the Pike River Coal mine.

The first explosion last Friday left the 29 miners trapped up to two kilometres underground, with only two managing to escape.

Officials insisted there were chances of rescuing them, but after Wednesday's second blast, police and mine officials said no one could have survived.

Flags flew at half mast and church bells tolled across the country as the government promised an independent inquiry into the  tragedy.

John Key, New Zealand's prime minister, said on Thursday "no stone would be left unturned" to find the cause of the blast.

Key travelled to Greymouth on the South Island to be with the men's bereaved families, saying they had legitimate questions about what caused the disaster.

"This is an inquiry that will leave no stone unturned to ensure that they get their answers as to why their men are not coming home," Key told reporters, adding: "The nation is grieving and mourning alongside them."

Earlier, Key acknowledged the families were desperate for the miners' bodies to be removed from the pit so they could be given proper funerals.

New Zealand's mining industry is small and generally considered safe. The tragedy deeply shocked the country and devastated families who - buoyed by the survival tale of Chile's 33 buried miners -nursed hopes that their relatives could emerge alive.

Mine operators said on Thursday they were determined to bring out the men's bodies.

"I still want them back and their families want them back and we'll be doing everything we can to make that happen. My love and support are with those guys," said Peter Whittall, the CEO of Pike River Coal.

But testing showed gas levels surged again soon after Wednesday's explosion and that another explosion was possible. Methane is leaking from the coal seam, and a smouldering fire somewhere in the mine is producing other toxic gas and a potential ignition source, officials say.

Mining experts were planning on expelling oxygen from the mine that could fuel a further explosion. One option was to pump inert gas into the mine to push the oxygen out, another was to seal the mine up to stifle any burning and then enter when it was safe, Whittall said.

Source: Agencies