Police, fire and ambulance crews were called to the small hillside mine of Gleision Colliery near Cilybebyll, situated in a valley in south Wales.
Seven men were initially stuck in the ventilation shaft of the drift mine but three managed to get out, police said.
“Seven people were initially in the mine at the time, three of them got out — with one taken to hospital. His condition is currently unknown,” said a South Wales Police spokesman.
“It is believed the other four remain inside. A rescue operation is under way. As you can imagine, it is quite a dynamic situation.”
Prime Minister David Cameron promised “every support” for the emergency services.
“My thoughts are with those missing and their family and friends at this very difficult time,” he added.
Police Superintendent Phil Davies said that there was water in the privately-owned mine although he refused to confirm reports it was flooded.
Only a handful of mines are still operating in Britain following the decline of the industry in the 1980s. Wales used to be a heartland of the British coal industry.
“This is the first mining disaster I have known for many years,” local councillor Arthur Threlfall said. “There are not many collieries left like there used to be.
“However, it is a very worrying situation and it has shocked a lot of people.”