The incident comes at a sensitive time, with Turkey in talks to end the conflict with the Kurds after the PKK militant group agreed to halt its decades-long armed struggle.

The conflict, which began in 1984, has cost more than 40,000 lives.

The deaths occurred when the Turkish troops were searching for the remains of a soldier who was shot dead by Kurdish fighters in the area in May 2022 and whose body was never recovered, Turkey's defence ministry said.

"Four other of our heroic comrades in arms, affected by methane gas, have died... bringing the total number of victims to 12," the ministry said Monday on X.

It did not explain the origin of the methane gas in the caves.

"During a search operation in a cave... previously known to have been used as a hospital... 19 of our personnel were exposed to methane gas," it had said on Sunday.

It did not give any information about the condition of the other seven soldiers who were exposed to the gas, but said a farewell ceremony for the 12 victims was held at an airport in southeastern Turkey.

The soldier whose body they were searching for was killed as Turkish troops were conducting Operation Claw Lock, a four-month sweep that began in April 2022 to root out Kurdish PKK militants holed up in caves along the border.

The methane incident occurred on Sunday as a delegation from the pro-Kurdish DEM party was meeting jailed PKK founder Abdullah Ocalan, who -- in a rare move -- extended his condolences over the deaths.

"This incident caused Mr Ocalan and all of us deep sadness," a delegation statement said, offering condolences "to their families and relatives".