The world needs to combat water waste, United Nations chief Ban Ki-Moon said Tuesday as he warned of growing water scarcity in years to come.

Speaking at the opening of a four-day Budapest Water Summit, Ban said nearly half the global population could be facing water scarcity by 2030, while demand could outstrip supply by 40 percent.

"We must address unsustainable use... Water is wasted and poorly used by all sectors in all countries," he said.

"We must use what we have more equitably and intelligently," he added, calling for closer cooperation between countries to meet the "common challenges" of water needs.

Nations should work together to ensure sustainable agriculture and food security, tackle climate change and ensure proper sanitation, the UN secretary-general said.

"Humanity has consumed more resources over the past five decades than over the previous five thousand years," added Hungarian President Janos Ader, the summit's host.

The summit was being held as part of the UN's designated International Year of Water Cooperation.