REPUBLIKA.CO.ID, WASHINGTON -- US Secretary of State John Kerry Tuesday urged both South Sudan President Salva Kiir and his sacked vice president turned rebel leader Riek Machar to stop fighting and open talks, the State Department said.

Kerry called both men and urged them to "accept a cessation of hostilities and begin mediated political talks," the State Department said in a statement.

Machar, whose forces have clashed several times in recent days with Kiir's troops, told Radio France Internationale: "Yes we are ready for talks. I have formed my delegation."

Kiir has said he too is ready for talks "without preconditions," in a meeting on Monday in Juba with US special envoy Donald Booth.

Booth was still in the South Sudanese capital on Tuesday to "attempt to secure final commitment" from both men to begin talks, the State Department said.

"The United States urges all parties in the crisis in South Sudan to implement an immediate cessation of hostilities," it said.

"This will offer critical humanitarian access to populations in dire need and open a space for a mediated political dialogue between the opposing sides."

South Sudan, which formally declared independence in July 2011 and is still the world's youngest country, has long been unruly but dramatically erupted into more intense violence last week.

Troops loyal to rivals Kiir and Machar are battling for territory and control of oil resources, and there have been reports of atrocities against civilians and of massacres targeted along ethnic lines.

The United States has been very active on the diplomatic front as tensions have risen, with President Barack Obama making several statements, Kerry working the phones and Booth in the region.

Some 400 Americans have been evacuated from South Sudan, and the US military deployed a special Marine Corps unit and aircraft to the Horn of Africa to prepare for possible further evacuations.

The UN Security Council on Tuesday approved the deployment of nearly 6,000 extra peacekeepers to the country, where government forces have recaptured the key town of Bor after a nearly week-long rebel occupation.

Thousands of South Sudanese have been killed in the violence, with reports of bodies piled in mass graves, the UN said Tuesday.

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