Rescuers are hoping the first of the 12 boys trapped with their football coach inside a flooded cave in northern Thailand will emerge safely within the next few hours.

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

The entire rescue operation is expected to take two to four days, depending on the conditions inside the cavern.

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

"Today is D-day," Mr Osottanakorn said.

A challenging underwater escape

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, some as young as 11, and their 25-year-old coach must dive through dark, narrow passageways, sometimes no more than 60 centimetres wide.

"We, the Thai team and the international team, will bring the Wild Boars home." ( Facebook: Thai Navy SEAL )

Experts considered such a challenging underwater escape a last resort, especially for people untrained in diving, as the boys and the coach are.

The dangers of the cave rescue were underscored on Friday when a former Thai Navy SEAL died after he passed out from a lack of oxygen.

Mr Osottanakorn said the mission was launched on Sunday morning because floodwaters inside the cave were at their lowest level in days.

The Thai navy SEALs, who have spearheaded the rescue effort, posted a photo on their Facebook page with a vow to bring the trapped team home.

"We, the Thai team and the international team, will bring the Wild Boars home."

The Wild Boars is the name of the football team.

The first boy is unlikely to be brought out before 9:00pm on Sunday (12:00am Monday AEST).

Rescue workers ramped up efforts on Saturday night in preparation to free the team, who have been trapped for two weeks.

Video shared by Anyawut Pho-ampai from Ruamkatanyu Foundation, showed workers drilling the walls of the cave to ease the divers' passage.

Police and security personnel in Chiang Rai also blocked off roads leading to the hospital.

A truck carrying oxygen tanks arrives outside the Tham Luang cave complex. ( Reuters: Tyrone Siu )

Rescuers at 'war with water and time'

Mild weather and falling water levels over the last few days had created optimal conditions for an underwater evacuation.

But a heavy rain shower hit northern Thailand overnight, worsening conditions at the cave.

On Saturday, Mr Narongsak said experts told him water from new rain could shrink the unflooded space where the boys are sheltering to just 10 square meters.

Earlier efforts to pump out water from the cave have been set back every time there had been a heavy downpour.

Mr Narongsak has described the mission as a "war with water and time".

"If we keep on waiting and the rains come in the next three or four days, our readiness will decrease," Mr Narongsak said.

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

A police officer raises a police tape as divers arrive at the cave complex. ( Reuters: Soe Zeya Tun )

Ambulances and helicopters at cave on stand-by

Thirteen fully staffed medical teams are stationed outside the cave — one for each of the 12 boys and their coach.

Each of the teams has its own helicopter and ambulance, as well at least one doctor, two nurses and a paramedic.

Medical staff involved in the mission say their first assessments will focus on the boys' breathing and signs of hypothermia.

They will also check for an airborne lung infection known as cave disease, which is caused by bat and bird droppings and can be fatal if it is untreated and spreads to other parts of the body.

After the initial assessment, the group will be driven by ambulances to makeshift helipads and airlifted to the Chang Rai hospital some 70 kilometres away, Major General Pramote Imwattana of the Army Medical Department said.

"It takes 10 minutes to get from our field medical tent to the helipad and about 15 minute in the air to the hospital," he said.

Videos of the boys after their discovery on Monday showed them looking frail, thin and exhausted.

But they appear to have remained in good spirits, and Mr Osottanakorn said on Saturday that the boys' health was the "best yet".

A row of ambulances are at the cave site ready to transport the boys and their coach to makeshift helipads. ( ABC News: Anne Barker )

Family wait for news of the boys' rescue

The families of the boys have been informed about the risky mission.

On Saturday they received handwritten messages from the children and their coach, which were delivered by divers who have made the 4 kilometre trip each way to where the group are stuck.

One of the boys, identified as Tun, wrote: "Mom and Dad, please don't worry, I am fine. I've told Yod to get ready to take me out for fried chicken. With love."

The parents also had short lettered delivered to the boys.

Many made it clear they are not angry at their sons for going into the cave after a practice game on June 23.

Nor do they blame the coach Ekapol Chatarawong for putting their children in danger.

"Thank you very much for taking care of our children," one of the messages from the parents read.

"You went into the cave with our children and you must get out with them. Take our children and yourself out with safety.

"We are waiting in front of the cave."

Relatives and friends pray at Mesai Grace Church in the northern province of Chiang Rai. ( Reuters: Soe Zeya Tun )

ABC/wires