No rescue mission will be launched until at least Sunday morning at the New Zealand coal mine where 29 men are missing after a blast, officials say.
Police say they "remain positive" that the men will be found alive, but the fear of a second explosion is preventing rescuers from entering.
New Zealand's PM John Key has said bringing the men out of the Pike River coal mine remains his top priority.
There has been no contact with the men since they were trapped on Friday.
Anguish and despair
Regional police commander Gary Knowles said on Saturday that tests had been carried out on samples of gas taken from the mine.
But the tests had to be suspended later because gas levels in the mine had risen.
Speaking at a news conference, Mr Knowles said he was not prepared to send any of the volunteer rescuers into the mine until it was proved to be a safe environment.
"We will still look for that window of opportunity to get underground and get those men out," he said.
"We still remain positive and we believe that once that window of opportunity opens, we are ready to go."
The prime minister, speaking at the same press conference, said his government was "doing what we can to make sure these 29 brave men are taken out of the mine and are here in one piece".
"This is a time of huge anxiety for the families and for the miners, so our heart and thoughts go out to them," he said.
"The big issue is balancing the rescue of the miners with the safety and security of those who undertake the operation. It's a difficult time but we're determined get them out alive."
The families of the missing men have been waiting for news at the surface.
But there is a lack of certainty over when the long-awaited rescue effort will be able to begin, the BBC's Greg Ward in New Zealand reports.
The Mayor of the nearby town of Greymouth, Tony Kokshoorn, praised officials for being honest about the situation, but said there was a sense of "absolute frustration" among relatives and friends of the miners about the lack of progress.
"There is anguish on their faces, the body language is showing real despair at the moment. You can see it building all the time, and this is where it really gets tough for everyone," he said.
The explosion at the Pike River mine is believed to have happened at about 1530 (0230 GMT) on Friday.
An electrician went into the mine at 1550 to investigate a power failure, and 1,500m (4,920ft) into the shaft he discovered the driver of a loader who had been blown off his machine. He then raised the alarm.
Mining experts said it could have been an explosion of methane gas, coal dust, or a combination of the two.
The missing workers range in age from a 17-year-old - believed to be on his first shift - to a 62-year-old.
Mr Knowles said that - based on information provided by the mining company - there were 24 New Zealanders, two Australians, two Britons and a South African national.
Britain's Foreign Office later confirmed that two Britons had been reported missing in the incident. It said both men were from Scotland.
Pike River Mine Chief Executive Peter Whittall said the mine was relatively small, so the men would have been working close together.
Each miner carried 30 minutes of oxygen, enough to reach oxygen stores in the mine that would allow them to survive for several days. The mobile phone of one of the men was said to be still working, but was not being answered when called.
Mr Whittall said there were also supplies of water underground and the temperature was about 25C.
He said that until the location of the missing men could be confirmed, the structure of the mine meant it was not possible to send supplies down "with any certainty as to where it's going".
Two workers who walked out of the mine have been treated in hospital for moderate injuries but have since been discharged.
Officials said one of the men had been very disorientated when he emerged. The other was able to speak to officials and said he believed he had been knocked over by a blast, causing him to briefly lose consciousness.