UN leader Ban Ki-moon and the UN Security Council are urging South Sudan’s president to release political detainees at the center of efforts to start a ceasefire in the country’s conflict.

UN leader Ban Ki-moon and the UN Security Council are urging South Sudan's president to release political detainees at the center of efforts to start a ceasefire in the country's conflict.

Ban said Friday he spoke with Salva Kiir on Thursday to call for the release of the opposition officials to boost hopes of ending battles in which thousands are feared to have died since December 15.

"South Sudan is at a crossroads," the UN secretary-general told reporters.

The 15-nation Security Council also demanded the release of the detainees in order to "create an environment conducive to a successful dialogue" between Kiir and his former vice president Riek Machar.

Forces loyal to Kiir and Machar are battling for control of key cities while peace talks are held in Addis Ababa.

The Security Council said both sides should halt the fighting and that Machar should agree to a ceasefire "without precondition."

Ban said the crisis in South Sudan is now "very dire," with an estimated 75,000 people now crowded into UN compounds across the country.

The UN leader said widespread human rights violations had been committed and he would send Assistant Secretary-General for Human Rights Ivan Simonovic to South Sudan this weekend to review evidence of abuses.

"Perpetrators of serious human rights violations will be held accountable," Ban said.