Rescuers are racing to free hundreds of workers believed to be trapped in tunnels at hydropower sites that were buried in mud by catastrophic flooding on the Nepal-China border, with the death toll climbing and increasingly desperate families searching for missing loved ones.The combined deaths in Nepal and China have surpassed 950, with more than 4,400 people missing, including 43 Australians.A private-sector association representing power companies said at least 900 workers were unaccounted-for at 12 hydropower projects damaged by the flash floods in Nepal's Rasuwa and Nuwakot districts.Nepal army personnel work during a search and rescue operation near the Trishuli Hydropower Project site in Nepal's Rasuwa district. (Reuters: Nepal Army/Handout)About 500 of those workers were believed to be trapped in tunnels. At least one expert said that some workers could have access to oxygen, but others warned that the chances of finding survivors were diminishing. As the rescue efforts intensified, four bodies were recovered from three mud-filled tunnels, Nepal's army said.Some workers managed to call for help when the disaster struck last Wednesday and others who managed to flee told authorities and relatives that many of their colleagues were still in the tunnels, said Mohan Kumar Dangi, president of the Independent Power Producers' Association, Nepal.It was unclear whether rescuers have had any contact with the trapped workers since the initial calls for help.Six days after the disaster, fears were growing that some might have suffocated, Mr Dangi said.Giant tunnels are built to control water flowRescue efforts were focused on people trapped in the gigantic tunnels built for controlling the flow of water."These tunnels are big enough to drive trucks through," said Jakob Steiner, a geoscientist at the University of Graz in Austria based in Dhaka, Bangladesh."And from what we understand, they have managed to get oxygen to some of those stuck inside and are trying to rescue them now."It's a race for time, though, as people need food and water."In the Upper Trishuli-1 Hydropower Project, in the hard-hit Rasuwa district, at least 576 people were missing, Mr Dangi said, adding that about 300 of them were believed to be trapped inside the project's tunnel.Nepal's paramilitary force personnel carry the body of a flash flood victim for burial at a mass burial site in Chitwan, Nepal. (AP: Dar Yasin)At the Trishuli-3 Hydropower Project, crews were working to descend about 15 metres into a tunnel in an effort to reach those believed trapped inside, according to Brigadier General Raja Ram Basnet, Nepal's army spokesman.Workers shovelling mud from a flood-ravaged power station in Dhading, Nepal, told The Associated Press that the station is a key connection for providing power to more than 20,000 people in the region.Families desperate for information about missing loved onesNepal's disaster agency said on Monday that 939 deaths had been counted in that country, with 3,925 people missing, including 592 foreigners. As of Sunday, Chinese authorities reported 16 deaths and 546 missing. Chinese state media said 261 of the missing were foreigners, including over 100 Nepalis.Australian Foreign Minister Penny Wong said there were great fears for the safety of 43 Australians still missing."We are urgently seeking to ascertain their whereabouts … we do hold grave fears, grave concerns for their safety," she told 7.30.Ms Wong said the department was able to confirm nine of the missing 43 Australians had crossed the border into China at the time of the disaster."What we don't have as yet, confirmed from any of the authorities, is any information about the 43 in total," she said."This is such a difficult time. It's incredibly distressing. "We are working to do whatever we can and to provide any information we obtain."An Australian Air Force C-17A carrying humanitarian supplies, equipment and personnel is set to travel to Nepal on Tuesday, as part of $8 million in humanitarian support announced by the federal government.People stand on remnants of a bridge that spanned across Trishuli river in Nepal's Dhading District. (AP: Niranjan Shrestha)Chinese media highlight government efforts to rebuild roadsOn the Chinese side, rescue and recovery efforts included rebuilding the road leading to the Gyirong Port border crossing that regulated travel between China and Nepal.Trucks carried giant rocks as workers tried to repave a portion of the road, state broadcaster CCTV showed on Monday, next to rapidly rushing waters, which have been made more powerful by days of rain.The road is now under 2 to 3 metres of water. The repaving effort was just a little over a kilometre from the border, CCTV said.Although some rescue teams have managed to reach the border, that represented only a portion of the force.Chinese state media coverage has focused largely on the official response, highlighting the deployment of firefighters, the People's Liberation Army and instructions from top leaders. Little is known about victims on the Chinese side.Scientists who studied satellite images said Wednesday's flash floods started after bedrock underneath a glacier high in the Himalayan region collapsed on Wednesday, taking part of the glacier with it. The rockfall was so extreme that it registered as a magnitude-5.2 seismic event.The rocks and melted water then swelled the rivers in the valleys below, causing a torrent of water that swept buildings, bridges and earth downstream in Tibet and Nepal.AP/ABC