India is considering plans to dramatically increase the water it draws from a major river that feeds Pakistani farms downstream, as part of retaliatory action for a deadly April attack on tourists that New Delhi blames on Islamabad, according to four people familiar with the matter.

Delhi "put in abeyance" its participation in the Indus Waters Treaty of 1960, which governs usage of the Indus river system, shortly after 26 civilians in Indian Kashmir were killed in what India called an act of terror. Pakistan has denied involvement in the incident, but the accord has not been revived despite the two nuclear-armed neighbours agreeing a ceasefire last week following the worst fighting between them in decades.

After suspending India's participation in the treaty, Indian Prime Minister Narendra Modi ordered officials to expedite planning and execution of projects on the Chenab, Jhelum and Indus rivers, three bodies of water in the Indus system that are designated primarily for Pakistan's use, six people told Reuters.

One of the key plans under discussion involves doubling to 120km the length of the Ranbir canal on the Chenab, which runs through India to Pakistan's agricultural powerhouse of Punjab, two of the people said. The canal was built in the 19th century, long before the treaty was signed.