The death toll from last week's flooding that hit Nepal's mountainous border region rose to 904 on Monday morning, with 4,274 still missing.

Aug. 31 (UPI) -- The death toll from last week's flooding that hit Nepal's mountainous border region near China rose to 904 on Monday morning, with 4,274 still missing, officials said, as work is underway to try and rescue hundreds feared trapped in hydroelectrical tunnels.

Nepal's National Disaster Risk Reduction and Management Authority issued the updated numbers at 9 a.m. local time, increasing the number of deaths by more than 100 and the number of missing by more than 1,200 from a day prior.

Among the missing were 592 foreign nationals, it said. On Sunday, State Department spokesperson Tommy Pigott told ABC News' This Week that 85 Americans remain unaccounted for.

Unidentified bodies were being kept at 21 hospitals in the area, the agency said.

The disaster management agency also said 267 people were injured, though 156 of them have been discharged from the hospital.

More than 20,500 security personnel have been mobilized in the search operations, resulting in more than 10,400 people, including 252 non-citizens, being rescued, it said.

The flooding occurred Wednesday morning and is believed to have been triggered by a massive glacial collapse. The U.S. Geological Survey said the deluge registered as a 5.2-magnitude earthquake.

Mass destruction has been reported throughout the region. Preliminary information, cited by the International Federation of Red Cross, indicates that at least 18,000 households have been affected. UNICEF, the United Nations' Children Fund, said at least 17,000 children have been impacted.

Among facilities damaged were 18 schools completely destroyed and another 20 that were damaged, UNICEF said.

The update comes as rescuers worked through the night to reach 933 workers the disaster management agency said is missing and believed to be trapped in hydroelectric tunnels.

Efforts are being accelerated at at least one of the tunnels, the Ministry of Energy, Water Resources and Irrigation said in a statement on Sunday.

"Water accumulated inside the tunnel is currently being removed, while preparations have reached the final stage for sending rescuers inside equipped with oxygen, medical supplies, communications equipment and cameras," the ministry said.

"The team plans to travel approximately 180 meters from the portal to a possible working space, where it will assess the actual conditions, determine the location of those trapped and evaluate their safety."