The families of more than 600 workers believed to be trapped in a vast network of mud-filled tunnels along Nepal's Trishuli River are clinging to hope their loved ones can be found alive.

Rescuers continued battling to reach them on Monday, using explosives to blast into the hillside before pumping air into the Trishuli-3A site - fearing a critical window to reach survivors is closing.

"I wish we could find him. I wish he would come home," Anushila Bhandari, wife of Aurus Bhandari, a 33-year-old engineer missing on a project site, told BBC Nepali.

It comes as the death toll from the flood disaster on the Nepal-Tibet border reached 939, according to the country's disaster management authority.

The agency also revised its estimate of the number of missing to 3,925, of whom 592 are foreign nationals.

Roughly one in seven missing people are believed to be workers at the hydropower plants. Nepalese Prime Minister Balendra Shah said on Monday that 279 people had been rescued from the sites.

The rescue operation is being spearheaded by an international team made up of Chinese and Nepalese military personnel, while experts from India, Korea and Australia are also involved.

"I still have a 50% hope that he is alive because there is an oxygen system inside," Sita Pandey, whose husband was working in the plant when the floods hit, told Nepalese daily Kantipur.

Nepal relies heavily on hydropower and has spent years developing the projects along its mountainous river systems. At least 11 projects either completed or under construction were damaged in the floods, authorities said.

Rescuers began pumping air into a tunnel over the weekend, but the families of those trapped say that should have been done earlier.

A team was able to get into a tunnel leading to one hydropower plant earlier on Monday, but withdrew after finding no one there.

"The longer it takes, the more our hope dies. This operation must move much faster because every second is meaningful to us," Jiban Khadka, whose relative is also trapped inside the plant, told Kantipur.

"It's a frustratingly slow operation," Khadka added.

Nepalese leader Shah said rescue operations had been "intensified", noting that 11,379 people had been rescued so far.

Another family member anxiously awaiting news is Priya Kharel, whose brother Milan was working as an engineer in the Upper Trishuli-1 plant when the floods hit.

"Those who returned from the hydropower site said there were people inside the tunnel," she told BBC News Nepali. "So there is hope that if the army reaches them quickly, they can be saved."

Australian engineer Arnold Dix, who has been helping Nepalese authorities with the underground search operations remotely, said rescuers had been struggling to find the tunnels.

"The actual signature of them has gone - everything is just missing. It looks like a lunar landscape," he told BBC Radio 4's World at One.

Once rescuers knew where the power stations were, Dix said they could then decide the best way to access them, which typically involves digging and "blasting boulders".

"There's boulders bigger than houses - I've never seen that before in my life. It's just an extraordinary operation," the former head of the International Tunnelling Association said.

He said he still has some hope for those trapped underground, given the protection the subterranean environment might have offered from the deluge above.

Meanwhile, in Chitwan, a district far downstream from where the floods first hit, makeshift burials were taking place after the Himalayan nation's river systems brought nearly 300 dead bodies to its banks.

The air was thick with the smell of decay, as men wearing white Personal Protective Equipment (PPE) including masks carried unidentified corpses into trucks.

Over the weekend, a large portion of the forest outside the city was dug up. Medical staff and volunteers worked to extract DNA from the unidentified remains.

Ganesh Aryaal, the chief district officer, told BBC Nepali that relatives will still be able to extricate the remains of their loved ones, once they are identified through DNA testing.

"We don't have enough freezers here... We had to choose this last alternative of temporary burial system. We are really overwhelmed," he said.

In Tibet, the official death toll is 16, with 546 reported missing, according to Chinese state media. Footage of the floods has been heavily censored in China.

Chinese authorities have imposed "administrative penalties" – which can range from warnings and fines to short detentions – on people speculating online that the casualty numbers are a lot higher than reported.

In Nepal, authorities have also ordered the removal of online content that they deem to be misinformation about the floods.