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Desperation rose on Friday as Nigerian rescuers worked by hand to dig out around 20 trapped miners, with fears they may not survive after four days buried in a collapsed pit.
At least one person was killed and six rescued with severe injuries after the mine collapsed on Monday following torrential rains, officials said earlier this week.
Ibrahim Audu Husseini, the spokesman for the SEMA relief agency in central Niger State, told AFP on Friday that the rescue operation was proving "delicate and more difficult than expected".
He said workers were having to use chisels and hammers to reach the trapped miners due to a lack of equipment at the site, which is in a remote district plagued by heavily armed gangs.
Husseini warned there were signs the miners may not have survived the collapse.
"The smell of the water seeping from the pit has turned foul, causing fear of the worst," he said.
"We can't conclude that the trapped miners are dead because no corpse has been recovered but the foul smell from the water in the pit is sending an alarm."
The exact number of trapped miners is not clear and officials have given conflicting accounts.
On Wednesday SEMA said the number was more than 30, while on Thursday police told AFP an engineer at the site had "confirmed that 20 persons were trapped", adding that an investigation had been launched.
SEMA spokesman Husseini said "the government doesn't have the required equipment for digging into the pit".
He explained that boulders had rolled down and covered the mine during the heavy rains.
Workers would usually use dynamite to break the rock apart, he said, but had ruled it out as they took the "utmost care not to endanger the lives of those trapped".
"It is a manual and tedious process that requires patience and care," he said.
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Minerals such as gold, tantalite and lithium are mined in the area.
Shiroro is one of several districts in Niger State terrorised by bandits, who raid remote villages in northwest and central Nigeria to loot as well as kidnap residents for ransom.
Six people were kidnapped in the area on Sunday and another 20 abducted nearby on Tuesday, according to SEMA.
Last year, the Niger State government banned mining activities in Shiroro and other districts due to insecurity and safety concerns.
But artisanal miners have continued to operate, seeking to raise money for food and essentials after bandit raids displaced many from their homes and farms.