Rescuers from China and Australia have joined the search for a youth soccer team missing in a Thai cave, as divers believe they're closing in on the spot where the group could be sheltering.

The international rescue operation - which includes more than 1000 people from Thai emergency services, alongside a US military contingent and British cave experts - has been ramping up its efforts since the 12 boys and their 25-year-old soccer coach disappeared during an outing in the caves in northern Thailand on June 23.

Divers are now closing in on elevated dry area, called Pattaya Beach, where they believe the missing boys may have taken refuge in the Tham Luang Nang Non cave system.

The boys pictured shortly before their disappearance. Picture: Facebook (Facebook)

The dry area is several kilometers from the entrance of the cave.

Water from the flooded cave has receded in recent days, allowing rescue teams to gain ground, according to Chiang Rai provincial governor Narongsak Osatanakorn.

Oxygen tanks have been installed at 25-meter junctures for divers to use, and a water pumping machine has also being deployed, added Osatanakorn.

Heavy rains had previously hampered the rescue operation, as the caves became extremely dark and muddy.

On Friday a team of six Chinese experts arrived at the site in Chiang Rai province, according to the Chinese Embassy in Thailand. The experts hail from the Beijing Peaceland Foundation, an organization with more than 100 rescue teams and experience carrying out similar operations in mountainous Myanmar and Nepal.

Thai soldiers walk out of the entrance to the Tham Luang Nang Non cave on Friday. Picture: AP (AP)

(Supplied)

On Saturday Australia also deployed a team of six experts from its national police's Specialist Response Group, according to the country's Minister for Foreign Affairs Julie Bishop. The group, which typically carries out land search and rescue operations, also has experience diving in flooded caves.

For now, divers continue to work at the mouth of the cave, and drilling is underway at several points in the cave complex, primarily to relieve the flooding.

Anxious wait for families

The boys, who range in age from 11 to 16 years old and a part of the same soccer team, have been missing for nine days.

A park officer sounded the alarm after spotting the boys' bicycles near the entrance to the off-limits cave complex.

The rescue operation continues days after the boys went missing. Picture: Getty (Getty)

The cave labyrinth is popular with tourists and for the first kilometer (0.6 miles) or so inside the cavernous entrance, limestone rock formations hug high ceilings, creating an almost amphitheater-like atmosphere.

Deeper inside, the passages narrow into places that locals warn it's not safe to go.

In the vigils that have grown with every day of fruitless searches, families and friends have prayed, made offerings, and held fast to the possibility of signs of life. Some, enduring the torture of such a long, silent wait, have collapsed in the mud with exhaustion, and been sent to a hospital.

"I feel I have just lost my heart when I found his bag, mobile phone and his shoes," said Sudsakorn Sutham, whose son, Prajak is among the 12 missing boys. "But all I can do is wait."

Rescue workers carry water pumping equipments into Tham Luang Nang Non cave on Sunday. Picture: Getty (Getty)

Rescuers try other entry routes

Late last week trekkers discovered a hidden opening deep in the jungle that may be a alternative point of entry to the cave system.

The natural chimney, which is around 1.5 meters in diameter and at least 22 meters (72 feet) deep, was found to the north of the opening of the cave, where the boys' bikes were found last Saturday.

Approaching the tunnels from above is "more promising," according to British rescuers at the scene, as it bypasses a passage blocked by flood waters.

Two British cave experts entered the chimney Friday morning, managing to descend to a depth of about 20 meters, before being relieved by another team. Though this is not the first natural chimney found on Pha Mee, it remains the most promising, according to search and rescue teams on site.

Specialist equipment is able to transmit the images from the climber's helmet-mounted cameras back to the support team at the top of the chimney. (Supplied)

However, surveying the area for other possible entry points is slow and time-consuming, as the terrain is tough going -- mountainous and heavily forested. Heat imaging drones are also being used to search the hilly terrain above the cave system.

Ten days after the boys went missing, rescue teams are remain optimistic that they closer than ever to finding them.