The rescue at the Tham Luang cave complex started at 10am local time.

Authorities flagged 9pm local time (midnight in Bendigo) as the soonest any of the boys will likely be rescued.

EARLIER: The area around a cave in northern Thailand where a dozen boys and their soccer coach have been trapped is being cleared in readiness for the rescue operation.

The announcement from authorities came as dark monsoon rain clouds 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 coach.

"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."

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

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.

Weather.com forecast sustained thunderstorms lasting through Sunday and Monday, with further stormy weather expected for around the next two weeks.

Narongsak Osottanakorn, a former provincial governor leading the effort said the "ideal time" for a rescue could come in the next two or three days.

"We're still at war with water and time," he told reporters.

- Reuters