KATHMANDU — Nepal's death toll from last week's Himalayan flash flood climbed to 903 on Monday, with 4,247 people missing, as Chinese and Indian tunnel-rescue specialists joined army crews trying to reach more than 900 workers believed trapped underground in about a dozen hydropower project tunnels blocked by mud and rock debris.

The revised tally from Nepal's National Disaster Risk Reduction and Management Authority more than doubles the missing count reported Sunday, when Nepal put the dead at about 750 and the unaccounted at roughly 3,000. The U.S. State Department said 85 Americans remain missing; Chinese state media put the number of foreign nationals unaccounted for in neighboring Tibet at 261, from 23 countries.

Under the tunnels

The disaster began Wednesday with a glacier collapse on the Nepal-Tibet border that registered as a magnitude 5.2 rockfall, sending ice, rock, mud and water down valleys on both sides. Nepal counted 903 dead and Chinese authorities another 16, with 546 additional people missing in Tibet. Of those missing in Nepal, 933 are hydropower workers and about 500 are employees of the country's Truck and Container Association.

"A large amount of debris has been deposited and that is posing us a big challenge to open the tunnels," Nepal army spokesperson Raja Ram Basnet told Reuters. Rescue images released overnight showed soldiers using torchlights inside a pit dug alongside a blocked tunnel and earth-moving equipment shoveling debris under floodlights.

Foreign help

Nepal has declined general foreign search-and-rescue assistance but accepted tunnel expertise from India and China. Beijing has dispatched nine tunnel-rescue experts, shared satellite imagery, mobilized more than 2,100 relief workers and allocated at least 220 million yuan, or $33 million, according to state broadcaster CCTV.

State Department spokesperson Tommy Pigott said on ABC's "This Week" that nine Americans have been safely evacuated and "we are currently unaware of any U.S. citizen deaths at this time." Washington activated $3.6 million in emergency humanitarian assistance Saturday. Malaysia's Prime Minister Anwar Ibrahim pledged $1 million and a search-and-rescue team on Sunday. Prime Minister Balendra Shah called the flood an "unprecedented and devastating natural disaster" and said nearly 21,000 security personnel are engaged in the response.

The climate question

Nepal's foreign minister called for a global response to climate change, saying "Despite having only 0.1% contribution to the global emissions, people of Nepal have become major victims" of warming. Scientists cited by CBS News cautioned that it is too early to link Wednesday's catastrophe directly to global warming, though they said such events grow likelier as temperatures rise, especially in the Himalayas, which are warming faster than most other regions.

Fresh flood-warning alerts went to residents' phones early Sunday after the Bhotekoshi River rose, prompting Chinese officials to notify their Nepali counterparts of higher upstream flows. Rescue work, suspended several times since Friday over weather and fears the barrier lake formed by the collapse could trigger new flooding, resumed Monday in improved conditions, said Narendra Pariyar, the district administrator of Rasuwa.