In pictures: Mission to rescue boys from Thailand cave underway

A rescue operation began Sunday to extract 12 boys and their football coach who have been trapped for more than two weeks in a cave in northern Thailand, the head of the mission said. The officials also said that the rescue operation could take up to two to four days.

"Today is the D-day. The boys are ready to face any challenges," rescue chief Narongsak Osottanakorn told reporters near the cave site.

Earlier, Thai authorities on Sunday said they plan to evacuate an area around a cave in northern Thailand where a dozen boys and their soccer coach have been trapped so that a "rescue operation" can take place.

The announcement came as dark monsoon rainclouds loomed over the mountainous north of the country early on Sunday, potentially heightening risks at the cave where rescuers were still waging a "war with water and time" to save the 12 trapped boys and their assistant coach.

The boys, aged between 11 and 16, went missing with the 25-year-old chaperone after soccer practice on June 23 as they set out to explore the Tham Luang cave complex in a forest park near the border with Myanmar.

A relative shows a transcript of a message from a boy of nicknamed Adun, one of the 12 schoolboys trapped in a flooded cave, at a prayer session at Mesai Grace Church, in the northern province of Chiang Rai, Thailand.

"Assessing the situation now, it is necessary to evacuate the area for the rescue operation," said Mae Sai police commander Komsan Sa-ardluan over a loudspeaker. "Those unrelated to the rescue operation, please evacuate the area immediately."

Following a relatively dry spell, fresh torrential downpours could pose a setback to rescuers who have struggled to drain the Tham Luang cave complex in the northern province of Chiang Rai.