From South Asia and to the Middle East, from Australia to California, rivers and aquifers that cross boundaries have become potent sources of friction.

Farmers squabble with city dwellers over irrigation rights while countries in river basins complain about pollution or water theft from upstream, as their neighbours build dams to siphon off flow from the watershed.

"Conflicts about water can occur at all scales," the UN warned ahead of the World Water Forum, which winds up in Istanbul on Sunday.

"Local-level conflicts are commonplace in irrigation systems, where farmers vie for limited resources," it said in a massive document, the third World Water Development Report.

"Conflicts also occur at the scale of large national river basins -- multistate Indian rivers such as the Cauvery and the Krishna -- or transnational river basins, such as the Jordan and the Nile."

"Water wars" for the time belong in the realm of conjecture.

In more than half a century, there have been only 37 cross-border disputes about water that have led to some form of violence, while some 200 treaties on water-sharing have been negotiated and signed.

Some of these initiatives have worked well.

They include the 1960 Indo-Pakistani treaty on sharing the water of the Indus, which has survived two wars between the two neighbours; the Mekong Committee, which has functioned since 1957 and swapped data throughout the Vietnam War; and the Nile Basin Initiative, launched in 1999 gathering all 10 riparian, or river-bank, states along the world's longest river.

But there are also treaties that remain a dead letter, especially in Africa, which has nearly a quarter of the world's cross-border river basins.

The risk of bloodshed over the stuff of life is a scenario taken seriously by many specialists.

Global warming may already be causing changes in rainfall and snowfall patterns, affecting river flow and groundwater recharge, and amplifying water shortages in countries that are already under stress, say scientists.

One of the feared trigger points is the Middle East, where Israel's policies of drawing water from the River Jordan and coastal aquifers are bitterly resented in the West Bank and Gaza.

In a message to the World Water Forum on Thursday, Palestinian Authority leader Mahmud Abbas accused Israel of flouting international law.

Palestinians had four times less water per capita than Israelis living in Israel, a consumption level that fell far below the World Health Organization's guidelines for minimum daily access to water, he said.

"Palestinians should not be forced to wait until a peace agreement is reached before (they are) allowed (their) rightful share of the transboundary water resources," he said.

Flavia Loures, senior programme officer for international law and policy on freshwater at the World Wildlife Fund, said governments urgently needed to set in place better mechanisms for resolving water disputes.

"We really need a stable cooperation now, before we come to the point where, due to climate change, competition for water resources becomes much stronger," she said.

Loures said a solution could be found in a UN pact that was signed in 1997 by more than 100 countries -- China, Turkey and Burundi demurred.

The accord, called the Convention on the Law of the Non-Navigational Uses of International Watercourses, requires parties to pledge "equitable and reasonable" use of water resources that straddle an international boundary.

But only 16 countries have ratified the convention so far, and 35 are needed before it becomes international law. France this month announced its intention to ratify. Loures said it was possible that the convention could become international law in 2011.

A European diplomat, though, said some countries, notably China, which have a river watershed, baulked at the convention.

"They fear it will entail interference in their internal affairs," he said.