The mission to free 12 boys and their football coach from the Tham Luang Nang Non cave site in northern Thailand is underway.

Thirteen foreign divers and five Thai Navy SEALs entered the cave at 10:00am (1:00pm AEST) on Sunday as dark monsoon rainclouds loomed over the mountains.

The rescue operation could take two to four days, with the first boy unlikely to be brought out before 9:00pm (12:00am AEST).

The head of the rescue mission and Governor of Chiang Rai province, Narongsak Osottanakorn, said the trapped team members were "very ready" to come out of the cave.

Each of the boys will have two divers to help them navigate the muddy water, strong currents and oxygen-depleted air as they attempt to make their way out of the cave.

The children and coach must dive through dark, narrow passageways, sometimes no more than 60 centimetres wide.

Mr Osottanakorn said it would take several hours before the first boy emerged from the flooded cavern where they have spent the past 15 nights.

An army commander involved in the mission said extracting the whole group could take three to four days and depended on the weather.

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