* Israeli air strikes exacerbate water shortages -UN
* UN aid agencies step up food, water deliveries
By Stephanie Nebehay
GENEVA, July 18 (Reuters) - The United Nations said on Friday it was stepping up emergency aid to Gaza, where Israel's military offensive has made water shortages worse and stoked fears of more sewage contamination and water-borne diseases.
On Tuesday, U.N. aid agencies and the International Committee of the Red Cross warned that after years of Gaza's water system deteriorating, damage from the attacks meant the whole coastal strip was facing a water crisis within days.
"We are still very concerned about the water supply in Gaza, about half of the population are without water supply at this time," U.N. Office for the Coordination of Humanitarian Affairs (OCHA) spokeswoman, Amanda Pitt, told a news briefing in Geneva.
Israel stepped up its ground offensive in the densely populated coastal strip with artillery, tanks and gunboats on Friday and declared it could "significantly widen" an operation Palestinian officials said was killing more and more civilians.
It followed 10 days of barrages against Gaza from air and sea and hundreds of rockets fired by Islamist Hamas into Israel.
Some 1,600 homes in Gaza have been destroyed or severely damaged, displacing nearly 10,000 people, Pitt said. More than 22,000 people have sought refuge in 24 facilities of the U.N. Relief and Works Agency for Palestine (UNRWA), she said.
In all, 258 Palestinians, most of them civilians, have been killed since the fighting began on July 8, Gaza officials said.
At least 59 Palestinian children are among the victims, U.N. Children's Fund's (UNICEF) Chris Tidey said. "Child casualties include 43 boys and 16 girls aged between 1 and 17 years," he told Reuters.
There have been two Israeli fatalities.
"Our plan is to distribute food for 85,000 people. We have already distributed emergency food for 20,000 since the conflict erupted, in addition to the 600,000 we regularly assist with food, together with UNRWA," U.N.'s World Food Programme's (WFP)Elisabeth Byrs.
Only 50 percent of sewage pumping and waste-water treatment systems are believed to be operational and 900,000 people lack any water supply, UNICEF's Tidey said.
"We are already scaling up water tankering to communities whose water has been completely cut-off, and provision of bottled water and hygiene materials," he told reporters.
Outbreaks of water-borne diseases are feared in the crowded unhygienic conditions in Gaza's summer heat, agencies said.
"Definitely we are worried about outbreak of disease, especially diarrhoea can occur in this part of the Middle East if there is no access to potable water and the sanitation system is not in order," World Health Organisation (WHO) spokeswoman, Fadela Chaib, said.
"For an infant or young child, diarrhoea can be a fatal disease... A child can die within hours if there is lack of water and re-hydration," she said.
(Reporting by Stephanie Nebehay; Editing by Louise Ireland)