* Ethnic cleansing happening, rape widespread - U.N.

* Warning fighting could escalate in dry season

* Court and protection force agreed but not delivered

(Updates with more quotes, detail)

By Tom Miles

GENEVA, Dec 14 (Reuters) - World powers can stop a "Rwanda-like" genocide in South Sudan if they immediately deploy a 4,000-strong protection force across the country and set up a court to prosecute atrocities, the head of a U.N. human rights commission said on Wednesday.

Africa's newest nation plunged into civil war in December 2013 after a long-running feud between President Salva Kiir and his former deputy, Riek Machar, exploded into violence, often along ethnic lines.

"South Sudan stands on the brink of an all-out ethnic civil war, which could destabilise the entire region," commission chief Yasmin Sooka told an emergency session of the U.N. Human Rights Council in Geneva.

Fighting was expected to escalate again now that the dry season had started, she said. Gang rape was happening on an "epic" scale, she added, citing cases of women being raped at a U.N. site in the capital Juba within sight of U.N. peacekeepers.

Washington and other powers called the one-day meeting after Sooka's commission reported this month that ethnic cleansing was already taking place in South Sudan, which only seceded from Sudan in 2011. urn:newsml:reuters.com:*:nL8N1DW4ML

Kiir has denied there is any ethnic cleansing and South Sudan's ambassador at the council, Kuol Alor Kuol Arop, said his country saw no need for the special session.

International pressure including the threat of sanctions has so far failed to halt the fighting in an oil-producing country at the heart of a fragile region, including Sudan, Ethiopia and the Democratic Republic of Congo.

The warring sides agreed to a set up a court backed by the African Union in 2015, but one has not appeared.

There was an urgent need for a tribunal "with a strong focus on command responsibility for atrocities," the U.N. High Commissioner for Human Rights, Zeid Ra'ad al Hussein, told the meeting.

South Sudan's government has also said it will allow a 4,000-strong regional protection force to bolster the U.N.'s existing peacekeeping mission there. But it has also not arrived and Sooka said there were fears it would not operate beyond the capital.

"We urge the immediate deployment of the 4,000-strong regional protection force for South Sudan... People all across the country asked that it not be restricted to the capital if it is to protect civilians across South Sudan," she said.

The forum is due to vote on a resolution later in the day that would extend the mandate of the U.N. human rights commission in South Sudan for a year.

(Reporting by Tom Miles; Editing by Stephanie Nebehay and Andrew Heavens) ((tom.miles@thomsonreuters.com; +41 58 306 2006; Reuters Messaging: tom.miles.reuters.com@reuters.net))