GREYMOUTH, New Zealand — New Zealand’s prime minister warned the nation to prepare for the worst as there remained no viable way to reach 29 coal miners missing since Friday when they were trapped by a blast so powerful it broke surface vents and blew dust across nearby valleys.

A bomb-disposal robot that was sent into the Pike River mine to assess conditions and learn the fate of the men short-circuited and failed today when it hit water. Replacements were being flown from the New Zealand army and from West Australia, said police superintendent Gary Knowles, the head of the rescue operation.

“Toxicity is still too unstable to send rescue teams in,” Knowles told reporters. “This is a very serious situation and the longer it goes on, hopes fade, and we have to be realistic.” Rescuers cannot enter the mine because toxic and potentially explosive gases are swirling in the tunnels and heat underground is believed to be coming from a smoldering fire.

The robot breakdown and the release of security camera footage showing the huge power of a blast last Friday sent relatives’ hopes for the men’s survival plummeting.