The eight -- two couples and their children -- had slept in one room at a hotel in Daman, a popular tourist destination in Makwanpur district about 55 kilometres (34 miles) from Kathmandu.
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"They were found unconscious this morning and airlifted to Kathmandu but died during treatment," police spokesperson Shailesh Thapa Chettri told AFP.
The families, from the southeastern state of Kerala, used a gas heater in their room to keep warm, a district official told AFP.
"We suspect they died of suffocation, but autopsy reports will confirm the cause," Chettri added.
India is Nepal's biggest source of tourists, making up some 16 per cent of visitors to the Himalayan nation.
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