due to

a lack of access to water, the extreme temperatures and regular sandstorms.

The United Nations has condemned the

seizure

by South Sudanese forces of the Heglig oil field in the disputed border area with Sudan. Ban Ki-moon said South Sudan's action had no legal validity.

"I urge both sides to exercise maximum restraint, return to the negotiation table and resolve their differences. I call on South Sudan to immediately withdraw its forces from Heglig. This is an infringement on the sovereignty of Sudan and a clearly illegal act."

Mr Ban also criticised Khartoum and said its forces should immediately stop shelling and bombing South Sudanese territory, and withdraw from disputed areas.

A US Black Hawk helicopter has crashed in southern Afghanistan. Four soldiers are believed to have been on board. An American official said an attack could not be

ruled out

.

Violence and controversy have continued to surround Bahrain's Grand Prix motor race scheduled for Sunday. A car carrying members of India's Formula 1 team was caught in street battles as police and demonstrators fought each other with tear gas and petrol bombs. Protesters have threatened what they called "days of rage" to

coincide

with the event, which the sport's governing body, the FIA, earlier insisted would pass peacefully.

You're listening to World News from the BBC.

A Tibetan rights group says that two more Tibetans have set themselves on fire in southwestern China in protest against Chinese rule. The Free Tibet group said the two young men staged the protest together in front of a