The UN Educational, Scientific and Cultural Organisation (UNESCO) on Wednesday published the first global map of cross-border aquifers with the aim of helping management of a precious and often-threatened resource.

The inventory comprises 273 aquifers that straddle internal or international boundaries.

Aquifers are sponge-like underground rock that hold water.

They exist on all continents but some, such as those in in North Africa, the Arabian peninsula and the western United States, were formed more than 10,000 years ago when the climate was more humid and are no longer being replenished.

Burgeoning populations and extensive irrigation are fuelling concerns for these aquifers and some experts fear the risk of future "water wars" between rival states.

The map can be accessed on (http://www.whymap.org/nn_354300/whymap/EN/Downloads/Global__maps/globalmaps__node__en.html?__nnn=true).