The wife of the former Maoist insurgent leader now serving as Nepal's prime minister died in the Himalayan republic on Wednesday after a long illness, her husband's physician said.

Sita Dahal married Pushpa Kamal Dahal, better known by his nom de guerre Prachanda ("The Fierce One"), more than half a century ago when both were teenagers.

Few details about their lives are on the public record, but Sita was by her husband's side when he launched a decade-long insurgency against the government in 1996.

Sita, 69, had long suffered from numerous ailments including diabetes and hypertension.

Yubraj Sharma, the prime minister's official doctor, said in a statement that she suffered a cardiac arrest on Wednesday morning.

"Despite resuscitation (attempts), she could not be revived," he said.

Nepal's civil war claimed more than 17,000 lives before a peace deal brought an end to the conflict in 2006.

Sita was serving as an adviser to her husband's Nepal Communist Party (Maoist Center), which entered mainstream parliamentary politics after the insurgency ended.

"Our entire party is shocked by the tragic death of Comrade Sita Dahal," the party said in a statement.

Sita "played a coordinating role as the parent of the entire party during the stormy years of the civil war, to resolve challenges, crises and disputes within the party", the statement added.

In neighbouring India, Prime Minister Narendra Modi said he was "extremely saddened" to learn of her death.

"I express my sincere condolences... and pray that the departed soul finds eternal peace," he wrote on Twitter.

The couple had four children, two of whom have since died.