The death toll from devastating floods on Nepal's border with Tibet has exceeded thousand the national disaster management agency said. More than 3900 people are missing, over 600 of them workers, believed to be trapped in mud-filled tunnels at hydropower facilities along the Trishuli River, the BBC reports.

Engineers continue pumping air into the tunnels. One rescue team managed to enter one of the tunnels, but was forced to retreat after finding no people there.

Nepalese Prime Minister Balendra Shah announced on Monday that 279 people had been rescued from the damaged structures. The search and rescue operation is being led by an international team, including military personnel from China and Nepal, as well as specialists from India, South Korea, and Australia.

Nepal relies heavily on hydropower and has been developing hydropower projects along the country's mountain river systems for many years. According to authorities, the flooding damaged at least 11 facilities, both existing and under construction.

Relatives of the trapped workers are expressing concern about the slow pace of the operation and note that air pumping into the tunnels should have begun earlier.

Shah stated that the rescue operation had been intensified, calling the flooding an extraordinary event and noting that risks in the Himalayan region are increasing amid climate change. He called for greater attention from the international community to the problem of such natural disasters.