A third blast erupted today inside the New Zealand mine where 29 workers were killed in an earlier explosion.

Pike River Coal chairman John Dow said today's blast happened almost exactly a week to the minute after the first explosion that led to one of the country's worst mining disasters.

Twenty-nine workers, including two Britons, were caught in last week's explosion and officials said there was no way they could have survived a second blast on Wednesday.

High levels of potentially explosive methane gas blamed for all the explosions has kept recovery teams from entering the mine to recover the workers' bodies.

Pete Rodger, 40, from Perthshire, and Malcolm Campbell, 25, from St Andrews, Fife, were among the missing miners.

No-one was injured in the latest explosion at the Pike River Coal mine on the South Island.

Mr Dow said the third blast was smaller than the earlier two, and that no-one was near the mine entrance when it happened.

He said it would not affect planning for an operation to enter the mine to recover the men's bodies. Officials have said it could take weeks or months to complete the recovery operation.

"The environment continues to remain unstable," Mr Dow said. "The plans we have in place will continue. We have expected this will always be a possibility."

Workers are installing a jet-powered engine used to extinguish fires at the mine site. It will pump carbon dioxide, nitrogen gas and water vapour into the mine's tunnels to expel oxygen that could fuel more explosions.

Once that is done, workers wearing breathing apparatus could enter the mine.

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 - had clung to hope that their relatives could emerge alive.

The country has had 210 deaths in 114 years in mines. New Zealand's worst mine disaster was in 1896, when 65 died in a gas explosion at a mine on the same coal seam as the latest tragedy.

The most recent was in 1967, when an explosion killed 19 miners in a mine near the Pike River site. A fire in a mine in 1914 killed 43.