KHARTOUM, Feb 16 (Reuters) - Cholera has been confirmed in two southern Sudanese cities in a rapidly spreading outbreak that has claimed 27 lives, U.N. officials said on Wednesday.

With cities full to the brim of returning southerners following a 2005 peace deal, the bare infrastructure cannot cope and two-thirds of the population drink unclean water, a carrier of the disease.

"It's particularly alarming in Juba, where a city of at least 250,000 depends to a great extent on untreated water from the Nile," said the U.N. children's agency (UNICEF) spokesman Ben Parker.

In less than two weeks, 1,433 cases have been reported and at least 27 people have been killed by the outbreak first reported in the town of Yei earlier this month, The World Health Organisation (WHO) said.

The first case tested positive as cholera this week, the WHO said.

Cholera causes rapid dehydration, which can lead to death and spreads quickly among close-knit populations.

"The outbreak highlights the woeful state of water supply in southern Sudan's urban centres," Parker said.

More than two decades of war have devastated south Sudan, killing 2 million people, mostly from famine and disease, and forcing more than 4 million to flee their homes.

With a peace deal in January 2005, thousands of southerners are returning home, straining already creaking facilities in the towns.