By Michelle Nichols

UNITED NATIONS, April 24 (Reuters) - The United Nations Security Council asked for an urgent investigation on Thursday into the ethnic massacre of hundreds of people in South Sudan's oil hub Bentiu and expressed its willingness to "take additional measures" if there are more attacks on civilians.

The 15-member council said in a statement it wants U.N. Assistant Secretary-General for human rights Ivan Simonovic to return to South Sudan as soon as possible to assess the recent escalation in violence.

More than 1 million people have fled their homes and thousands of people have been killed since fighting erupted in December between troops backing President Salva Kiir and soldiers loyal to his sacked deputy, Riek Machar.

The fighting has exacerbated ethnic tensions between Kiir's Dinka people and Machar's Nuer. Negotiations between the Kiir government and rebels loyal to Machar have failed to advance since the Jan. 23 signing of a ceasefire that never took hold.

"The members of the Security Council indicated their willingness to take additional measures should attacks on civilians and violations of the (Jan. 23) Cessation of Hostilities Agreement continue," it said in a statement.

Security Council members are considering sanctions on South Sudan's warring parties and U.N. peacekeeping chief Herve Ladsous demanded on Wednesday "serious consequences" be imposed to force an end to the violence.

Ladsous and Simonovic briefed the council late on Wednesday.

The council demanded that Kiir, Machar and other leaders "demonstrate leadership by publicly stating that any and all attacks on civilians are unacceptable, committing to hold accountable those who order such attacks or carry them out, immediately ending the violence, and returning to the inclusive peace process."

South Sudan's government said on Thursday it would free four high-profile political prisoners facing treason charges, meeting a demand by rebels in the faltering peace process.

'OUTRAGEOUS' ATTACKS

U.N. Secretary-General Ban Ki-moon called East African leaders on Thursday to express support for their attempts to mediate a South Sudan peace deal through the Intergovernmental Authority for Development regional bloc, U.N. spokesman Farhan Haq said.

The United Nations accused the rebels of hunting down men, women and children last week in a hospital, church and mosque in the capital of the oil-producing Unity state and then killing them based on ethnicity and nationality.

After the rebels seized Bentiu, Dinka residents of Bor town in Jonglei state attacked a U.N. base where about 5,000 people, mostly Nuer, had sought shelter. Fifty-eight people were killed and 98 were wounded, including two Indian peacekeepers, the United Nations said.

On Thursday, U.S. Ambassador to the United Nations Samantha Power condemned the deliberate targeting of civilians as "outrageous" and said South Sudan's leaders must take immediate steps to end the violence and participate in peace talks.

"Failure to take bold action now very well could push South Sudan into a cycle of retaliatory ethnic killing, a deepening civil war, and an even more devastating humanitarian catastrophe," Power said in a statement.

"The Security Council must take action against those who continue to undermine peace efforts and swiftly create a sanctions regime targeting spoilers of the peace process and those responsible for atrocities."

Power said the government of South Sudan needed to "work far harder" to stop attacks on the United Nations. Tens of thousands have sought refuge at U.N. bases across the world's youngest nation, which seceded from Sudan in 2011.

China, the biggest investor in South Sudan's oil industry, said it would "conscientiously participate" in Security Council discussions, but stopped short of saying whether it would support sanctions. (Additional reporting by Ben Blanchard; Editing by Toni Reinhold)

Our Standards: The Thomson Reuters Trust Principles.