Emergency rescue teams gather in the staging area as they continue the search for a young soccer team and their coach believed to be missing in a large cave on Tuesday. (AP photo)

NAE SAI, Chiang Rai: Intensive efforts to rescue 12 boys and their soccer coach who have been trapped inside a flooded cave in northern Thailand for three days hinge on pumping out water so that navy divers have headroom to operate, Interior Minister Anupong Paochinda said on Tuesday.

Gen Anupong told reporters that Navy Seal divers leading the search for the missing are seriously handicapped by muddy water that filled some chambers of the cave to their ceilings. He said the divers could proceed only when enough water has been pumped out so there is breathing space between the water and the ceiling. The divers will also soon start using special oxygen tanks that provide longer diving times, he said.

Gen Anupong said that Tuesday's goal was to be able to "reach the kids'' and that rescuers would be working night and day.

"I want to confirm to the media that the Seal team will be working nonstop because it's already dark here too,'' he said. "So night and day doesn't make a big difference. They'll just need to rotate.''

Prime Minister Prayut Chan-o-cha called a meeting with chiefs of the armed forces at Don Mueang airport to discuss the rescue efforts after he landed in Bangkok from a European trip on Tuesday.

About a dozen navy divers and other rescuers re-entered the partly flooded cave on Tuesday morning to search for the boys, aged 11-16, who have been missing since their coach took them to the Tham Luang Nang Non cave complex on Saturday after a practice match.

Divers have been seeking a way forward through the chambers of the cave complex, but have been forced to suspend their search several times. The authorities have also been seeking alternative ways in, using helicopters and search parties on foot to find holes that might exist in the ceilings of other parts of the cave.

Rain that fell overnight increased the difficulty of exploring inside the cave. However, the initial chambers near the cave's entrance are dry, and a power line was extended inside to provide light and ventilation and help the divers communicate with those outside.

The boys and their 25-year-old coach entered the cave in Chiang Rai province late Saturday afternoon. A mother reported that her son did not return from soccer practice that day, setting off the search.

• Underwater robot, airborne drones aid cave search

Relatives of the missing boys and others performed a ritual Tuesday morning calling for those who are missing. They played drums and gongs and two relatives held fishing nets as symbols to fish out lost spirits from the cave. Organiser Jiratat Kodyee said the ritual was a traditional way of showing support for the boys' families.

"We hope that the water level has gone down, but we will have to see,'' Navy Lt Naponwath Homsai said Tuesday morning. "Today we will try to find passages under the water that hopefully will lead to other chambers.''

However, Tuesday's initial search mission had to be suspended.

The cave complex extends several kilometres and has wide chambers and narrow passageways with rocky outcrops and changes in elevation. Still, officials have said they are hopeful the boys found a safe space away from the floods.

Rising waters Monday evening first frustrated efforts to search farther in the cave, and the efforts were halted temporarily. During the night, rescue teams and electricians extended a power line 1km and communication lines into the cave.

"We hope this would provide lights for work and fans for ventilation for the Seal team,'' Chiang Rai governor Narongsak Osottanakorn said. "Also, it means we can use electric engines to pump water out from the cave as well.''

Parents waited overnight in tents outside the cave entrance as rain poured. Medics sat in a tent nearby, and bicycles, backpacks and soccer cleats the boys left behind remained at the entrance.

At a prayer session Monday evening, some of the boys' relatives walked inside the cave entrance, where their cries echoed off the walls. "My son, come on out! I am waiting for you here!'' one woman cried. Another kneeled down near the bicycles and prayed, asking "Where is my child?''

Namhom Boonpiam, whose 13-year-old son Mongkol is among the missing, said she had been waiting at the entrance since Saturday night.

"I haven't slept and I hope that all of them can come out, all safe and sound,'' she said. "My son is a strong boy. I still have hope.''

Authorities have said footprints and handprints were found inside the cave complex, and that tourists trapped there by past floods have been rescued after the waters receded.

Officials are hopeful there are still safe spaces in the cave complex despite the flooding, Chiang Rai deputy governor Passakorn Bunyalak told a Monday news conference.

"We're confident that the kids should still be in good condition,'' he said, noting that rescuers had seen nothing inside the cave to indicate otherwise.

Getting farther into the cave has required lots of oxygen and special diving skills, which would also complicate rescue efforts once the boys are found, Passakorn said. He said divers might have to first bring in food and the boys might need to wait out the flood or learn the basics of scuba to get out.

The cave, cut into a mountainside near the border with Myanmar, can flood severely during the rainy season, which runs from June to October, and there are warnings against exploring then.