Six of India's coal-fired power plants do not have enough stocks to weather even a "small" disruption in supplies, with fuel reserves sufficient for only up to two days, the country's largest power producer has warned.

State-run NTPC has asked the power ministry to ensure more coal is sent to the six plants - which have a combined capacity of 16,840 megawatts or 15% of India's total energy capacity from coal-fired plants - particularly as the incoming monsoon rains could disrupt supply deliveries.

Coal is used to generate more than half of electricity. The government is struggling to provide enough power to meet rising demand amid blackouts.

"With the ensuing monsoon, it will become more difficult to replenish the coal stocks and in case of even a small disruption, the total power generation at these stations will be adversely affected," NTPC Chairman Arup Roy Choudhury said in a July 14 letter to the Ministry of Power.

Stocks at three of the six plants would last less than one day, Choudhury wrote in the letter, which was obtained by Reuters. Choudhury was not immediately available to provide an update on stock levels on Thursday.

State behemoth Coal India, the world's largest coal miner, has failed to raise its output fast enough, forcing government to import millions of tonnes of the black rock for its thermal power plants.

Insufficient reserves are particularly problematic this year. Weak monsoon rains have limited the output from hydropower plants - which provide about a fifth of India's energy - putting added pressure on the thermal plants to raise output.

But any onset of rains would also threaten to disrupt supplies of coal from mine to power station, hitting output.

Last week Power Minister Piyush Goyal urged power companies to raise thermal coal imports after saying that 26 out of 100 coal-based power plants in India had "super critical" stocks - or only enough to meet requirements for less than four days.

As of July 14, a total of 44 plants, including the super critical ones, have "critical" coal stocks - only sufficient for less than a week - with the majority of these in the state of Maharashtra, Central Electricity Authority data shows.