Nepal's former prime minister Sushil Koirala died in Kathmandu aged 77 after a bout of pneumonia, his doctor told AFP on Tuesday.

Koirala, who was Nepal's premier from February 2014 to October 2015, faced heavy criticism for his government's sluggish response to a massive earthquake that devastated the Himalayan nation last April, killing nearly 9,000 people.

He suffered from chronic bronchitis and was diagnosed with pneumonia last week, his doctor, Kabirnath Yogi, said.

"He was taking medicines and was even showing improvements yesterday. But at 11 last night his condition suddenly deteriorated," said Yogi, an associate professor at Kathmandu's Teaching Hospital.

"He passed away at 12.50 before the ambulance arrived," Yogi said.

Koirala headed the country's biggest political party, the Nepali Congress, and was tasked as premier with writing a long-delayed new constitution to complete a stalled peace process and bring stability to the war-torn nation.

The constitution, which was passed in September, was meant to cement peace and bolster Nepal's transformation from a Hindu monarchy to a democratic republic after a decade-long Maoist insurgency.

But it has instead sparked violence, with more than 50 people killed in clashes between police and ethnic minority demonstrators protesting against the charter, which they say leaves them politically marginalised.

When he was in his 30s and exiled with his family in India, Koirala was involved in hijacking a plane believed to be carrying boxes of cash, which he and his relatives wanted to use to fund the Nepali Congress.

He spent three years in an Indian jail over the crime, which was masterminded by senior Congress leader and relative G.P. Koirala.

At the time, Nepal was run under a party-less "panchayat" system overseen by a monarchy that had overthrown a short-lived democratically elected government in 1960.

Koirala had a long history of poor health, undergoing radiotherapy for lung cancer in 2014 and surgery for tongue cancer 10 years ago.