Naval experts hailed the two-month mapping exercise as a breakthrough in a quarrel that goes back to British rule and has prevented the exploration of gas and oil deposits in a key economic zone in the Arabian Sea. Hydrographers aim to produce a common map and single set of data from which a mutually acceptable border can be established.

The main sticking point is whether the line should be drawn on the eastern bank of the estuary, as Pakistan demands, or, as India contends, mid-stream based on a principle of international law that defines borders along the lowest points of a riverbed. Sir Creek, in the Rann of Kutch region dividing the Indian state of Gujarat and the Pakistani province of Sindh, has been the subject of nine rounds of bilateral talks since 1969.

The uninhabited salty marsh represents just one of several unresolved border disputes — the most important being Kashmir — as India and Pakistan embark on a new phase in a three-year peace process that has yet to yield tangible results.

If the argument over Sir Creek is to be finally resolved, analysts say, it will require the spirit of co-operation that emerged over the weekend during talks between Pranab Mukherjee, the Indian Foreign Minister, and President Musharraf of Pakistan.

The Islamabad meeting yielded an agreement to hold an anti-terrorism summit in March and to expedite a resolution of another border dispute over the Siachen Glacier, a Himalayan wasteland dubbed the world’s highest battlefield.

“It’s no good sticking to old, established positions. There has to be give and take. This time, both governments seem keen,” Vice-Admiral P. J. Jacob, former vice-chief of Indian naval staff, said. “It is in everyone’s interests to resolve this.”

Sir Creek was named after the British representative requested to mediate between the Rao of Kutch and the ruler of Sindh in a dispute over firewood. Disagreement over the maritime border led to a minor Indo-Pakistan war in April 1965. It was again at the centre of a stand-off in 1999 after the Indian Air Force shot down a Pakistan Navy surveillance aircraft.

Fishermen from both countries are regularly arrested because they are unclear about the demarcation. Pakistan released 115 Indian fishermen over the weekend as a goodwill gesture, but up to 400 are still thought to be captive. Under the UN Convention on Law of the Seas, the countries have until 2009 to settle the boundary or the continental shelf, thought to hold rich gas and oil deposits, may be opened up to exploration by third parties.

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