United Nations - President Barack Obama will attend a U.N. summit on Sudan this month, U.S. officials said on Wednesday as they stepped up a bid to head off conflict there before a referendum that could split the African nation.

U.S. Ambassador to the United Nations Susan Rice said Obama had accepted an invitation from U.N. Secretary-General Ban Ki-moon to a September 24 meeting on Sudan on the margins of the annual General Assembly gathering of world leaders.

The meeting in New York will bring together leaders from U.N. Security Council and other interested countries as well as United Nations, African Union and World Bank representatives.

It is expected to focus on a January 9 referendum among the people of semi-autonomous southern Sudan on whether to become an independent country, as well as on the seven-year-old conflict in Darfur, western Sudan.

Earlier, U.S. Secretary of State Hillary Clinton said Sudan was a "ticking time-bomb" ahead of the vote and that the international community must redouble efforts to head off violence there.

The State Department said Clinton had telephoned Sudanese Vice President Ali Osman Taha and southern leader Salva Kiir on Wednesday.

It also said that Scott Gration, U.S. special envoy for Sudan, would make a new trip to the region on Thursday to pursue talks on preparing a peaceful referendum.