By Michelle Nichols

UNITED NATIONS (Reuters) - The United States and Britain pledged on Thursday to support African Union efforts to set up a hybrid court to try war crimes committed during South Sudan's civil war after the country's leaders backed away from their commitment to the tribunal.

South Sudan President Salva Kiir and his deputy and formal rival Riek Machar called on the international community on Tuesday to reconsider creating such a court and instead support a mediated peace, truth and reconciliation process.

Kiir and Machar committed to an African Union hybrid court under a peace deal they signed in August that says no government or elected officials could be immune from prosecution. Some fighting continues in the oil-rich country.

United Nations sanctions monitors told the Security Council in January that Kiir and Machar qualified to be sanctioned over atrocities committed during the war.

"The United States will continue to make every effort to both support the African Union in its establishment of the hybrid court and to promote reconciliation among the people of South Sudan," Deputy U.S. Ambassador to the United Nations David Pressman told the U.N. Security Council.

Kiir's sacking of Machar as his deputy in 2013 sparked a brutal conflict and renewed fighting between Kiir's Dinka and Machar's Nuer people. More than 10,000 people have been killed and the world's youngest country, which gained independence from Sudan in 2011 with strong U.S. support, has been torn apart.

"Part of the challenge of rebuilding a nation lies with pursuing both justice and reconciliation, but not one at the expense of the other," said Pressman. He added that Washington was surprised and disappointed by Kiir and Machar's call to halt plans for the tribunal.

The United States has threatened to impose sanctions on Kiir and Machar if they backtrack from the peace agreement.

Britain also reiterated its support for the hybrid court.

"Accountability and reconciliation are not mutually exclusive. South Sudan needs both to promote long-term stability," said a spokesman for Britain's U.N. mission.

(Reporting by Michelle Nichols; Editing by Dan Grebler)