India in April announced it was suspending the Indus Water Treaty in the lead up to armed conflict between the nuclear-armed neighbours and following a deadly militant attack in India-administered Kashmir that New Delhi blamed on Pakistan, which denied involvement.

Pakistan has said any attempt to stop or divert the flow of water into the country would be considered an "act of war".

Foreign Minister Mohammad Ishaq Dar told foreign diplomats on Friday that Pakistan's water treaty commissioner had written a letter to his Indian counterpart over "unusual, abrupt variations" observed in the flow of the Chenab river from December 7-15, similar to changes detected in April and May.

"These variations in water flows are of extreme concern for Pakistan, as they point to unilateral release of water by India," he said, according to a ministry statement.

India's foreign ministry did not immediately respond to request for comment.

"India has released this water without any prior notifiation ... as required under the Treaty," Dar added, saying it represented a "weaponisation of water".

"India's manipulation of water, at a critical time of our agriculture cycle, directly threatens the lives and livelihoods, as well as food and economic security, of our citizens," Dar said in the speech to the international diplomatic corps in Islamabad.

"What we are witnessing now is material breaches by India that strike at the heart of the Indus Waters Treaty" signed in 1960, he added.

India suspended the treaty following a deadly attack on tourists in Indian-administered Kashmir on April 22 that killed 26 people.

New Delhi blamed Islamabad for backing the attack -- which Islamabad denies -- and said it was suspending the water treaty in response.

The two countries in May then engaged in intense tit-for-tat drone, missile and artillery exhanges which left nearly 70 people, including dozens of cilians dead on both sides.

India has said it is building additional dams on the Chenab that could over the long term further restrict flows to Pakistan, threatening the livelihoods of millions of farmers as well as drinking water supplies in the rapidly growing country.

The 1960 treaty, negotiated with the help of the World Bank over several years, ensured "equitable use" of six tributaries that feed the Indus river system.

India's suspension letter, sent to Pakistan in April, also cited "fundamental changes to the circumstances" since the deal was signed, including "population dynamics" and a "need to accelerate the development of clean energy".