OUAGADOUGOU – Rescue teams pumping water from a flooded Burkina Faso zinc mine in which eight workers have been trapped for almost a month are drawing closer to a refuge chamber where they may have sought safety, the government said on Wednesday.

The six Burkina Faso nationals, one Tanzanian, and one Zambian, have been missing since heavy rainfall caused flash floods at Canada-based Trevali Mining Corp's Perkoa mine on April 16, forcing it to suspend operations.

The company said that while most workers underground were able to safely evacuate, the eight missing were below Level 520, which is 520 metres from the surface, at the time of the flooding.

There are two refuge chambers designed in case miners become trapped, but it is not known whether any of the missing workers had been able to reach them.

One is located at Level 570, stocked with three weeks' worth of food and water, and a second smaller one sits much deeper in the mine, which reaches 710 metres below the surface.

Burkina Faso's government spokesman Lionel Bilgo on Wednesday said that over 38-million litres of water had been removed, leaving 10 metres of water above the first refuge chamber.

"The air vent is still working so that allows us to keep hope," Bilgo told reporters on his way out of a council meeting.

"It is a race against time," he said.

Trevali confirmed Bilgo's update, adding that although a compressed air supply line to the chamber was still holding pressure, there was no way of knowing whether it was functioning.

"Key factors of survival are availability of oxygen and how the people use the water and any food reserve," it said in an emailed comment.

Distraught family members have been meeting each day at the site in central Burkina Faso's Sanguie province for updates and moral support.

"It is not easy," Antoine Bama, the brother of one of the missing workers, told Reuters.

"The more the number of days increases the more anguish there is," he said via telephone.

Burkina Faso's government has launched a judicial probe into the incident and said last week that mine managers would be banned from leaving the country while investigations were underway.

Trevali is also working to determine the cause of the accident.

"We initially underestimated the scale of the catastrophe" said Ditil Moussa Palenfo, country director of Nantou Mining, the Trevali entity that owns Perkoa. "There is still a lot to do" to reach the refuge chamber, he added.