(Reuters): U.S. floats U.N. sanctions to press on South Sudan peace

(Adds details on process, quotes)

WASHINGTON Aug 19 (Reuters) - The United States on Tuesday put pressure on political leaders in South Sudan by announcing it was consulting with other countries about imposing United Nations sanctions on anyone who undermined the peace process in the war-hit east African state.

Susan Rice, President Barack Obama's national security adviser, said in a statement that Washington was proposing such sanctions "if an agreement is not signed by the government within 15 days and a cease-fire is not implemented promptly by all parties."

On Monday, South Sudan President Salva Kiir declined to sign a peace deal proposed by regional leaders, saying he required more time.

The country, which gained independence from Sudan in 2011, descended into chaos in December 2013 when a political row between Kiir and his deputy Riek Machar spiraled into armed conflict that reopened ethnic fault lines.

Seyoum Mesfin, the mediator for IGAD, the East African bloc leading the talks, said Kiir's side required two weeks before signing the peace deal that had been already accepted by the South Sudanese rebels.

"In the next 15 days, the government will come back to Addis Ababa to finalize the peace agreement," Seyoum said. There was no immediate comment from the government.

The U.S. State Department said on Monday it deeply regretted the government decision and urged it to sign within 15 days.

Numerous rounds of negotiations have failed to end fighting that has killed over 10,000 people and displaced more than 2 million, with both sides engaging in a war of attrition despite signing ceasefire deals.

Last month, IGAD set Aug. 17 as the deadline to end the drawn-out talks, outlining proposals for ending the fighting.

The proposal designates a 30-month transition period with Kiir as president, with a first vice president post allocated to the rebels, IGAD said.

Elections would be held two months after the close of the interim period. Both Kiir and Machar would be eligible to run. (Reporting by David Storey; Editing by Bernard Orr)