Having failed to find any sign of 12 boys and their soccer coach during a week-long search of flooded cave complex in northern Thailand, rescuers have held out hope that the group found safety on a rocky mound in one of the chambers.

Divers from the Thai navy's elite SEAL unit were groping their way through the murky waters filling passages of the 10km underground maze of the Tham Luang cave, as the rescue effort turned increasingly desperate.

International rescue teams, including 30 US military personnel, have joined the search for the junior soccer team, which went missing last Saturday, but the rescuers have been hampered by heavy rain.

Divers were still some way from the potential safety spot, called "Pattaya Beach" after one of Thailand's best known tourist destinations.

"The distance we've still got to go is probably two to three kilometres," the governor of Chiang Rai province, told reporters.

In the meantime, a police search party has entered the cave from the surface, having drilled a 50-metre long shaft from the mountainside.

Survival boxes, filled with food, water and torches, had been dropped down the shaft yesterday, but by today it was wide enough for rescuers to be lowered into the cave.

A Buddhist monk led prayers for the missing boys near the mouth of the cave

Deputy Police Chief Wirachai Songmetta said that search party has, as yet, been unable to reach the underground chamber that could be the best hope of survival for the boys.

Aside from bicycles and soccer boots left near the cave's entrance, and some handprints seen on the walls, the searchers have found no trace of the boys, aged between 11 and 16, or their 25-year-old assistant coach.

Still, rescuers believe the boys have a chance of surviving, and could have access to fresh water inside the cave, and medical teams were practicing drills to treat survivors, and airlift them from a make-shift helipad.

Relatives kept a vigil, with prayers led by a Buddhist monk, near the mouth of the cave.

"I am still hopeful and hope the children will come out safely," an uncle of three of the missing boys said.