In Northern Thailand, the whereabouts of 12 boys and their football coach thought to have disappeared in a cave complex are still unknown. Rescue attempts stalled on Thursday, after heavy rain forced workers to abandon their search. While Thai authorities maintain they are still hopeful of finding the group alive, it's been six days, and time is running out. Tony Cheng reports from Tham Luang Cave in Northern Thailand.

Dark rain clouds hang over the mountains on the Thai Myanmar border. Nestled between is Tham Luang cave where the twelve boys and their coach disappeared. Rain has hampered the search and rescue operation throughout and now flooding is higher inside the caves than its been all week. Rescue workers inside forced to suspend operations until the water level could be reduced.

TONY CHENG THAM LUANG CAVE, THAILAND "The water pumping system that they brought in from Bangkok last night failed yesterday in heavy rains. That's put back the search and rescue effort by at least 12 hours. They're trying to bring a more permanent system in now, but of course with every new effort, it takes time, and the longer it takes, the longer the boys and their coach remain stuck inside."

Help has arrived, however. US Special forces, specialists in rescue operations, with equipment from their base in Okinawa, are now on site.

CAPTAIN JESSICA TAIT PUBLIC AFFAIRS OFFICER, US ARMY "We're really here in an advise and assist role, and the royal Thai govt. asked us to come and provide that capability and knowledge, and hopefully we can support our ally and friend, Thailand."

And on Thursday the focus shifted from underground to the air. Choppers searching for chimneys in the jungle that might give access to the caves below. Drones have been flying too. These engineers are trying to survey the jungle floor to try and find the best access points to the cave complex. 

DR THANA SARANVECHPHAN SPECIALIST, PTTEP "To survey the terrain, on top of the cave, and try to generate and develop the 3D map."

A lot going on as the rescue workers try to move forward. But hope is starting to waver and the reality that this is a search that has so far offered up no clues. Tony Cheng CGTN Tham Luang Cave Northern Thailand.