"Samples we took do indicate that we've got a heating of some sort underground, that means that there's some combustion of material generating the gases that go with that," Pike River chief executive Peter Whittall said.
New Zealand Mine Rescue chief Trevor Watts described the mine, where 29 miners have been trapped since Friday, as a "gun barrel" and said the areas of greatest concern "are in an explosive atmosphere".
Police say they had "no idea" when it would be safe for rescuers to try to reach the men at the Pike River colliery, who have not been heard from since the blast on Friday.
"This is not a quick fix, we're into day two, we have no idea how long this will take but we are still focused on bringing these guys out," police commander Gary Knowles told reporters.
Relatives of the five foreign nationals among the 29 to New Zealand are to be flown in. Local people attended church services Sunday to pray for a successful rescue.
Tearful family members, who had been kept away from the disaster site since Friday's explosion, were taken to the scene for a two-hour visit Sunday to view rescue reparations.