UNITED NATIONS (Reuters) - U.S. President Barack Obama will attend a U.N.-organized summit on Sudan on the margins of the U.N. General Assembly on September 24, U.S. Ambassador to the United Nations Susan Rice said on Wednesday.

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

The meeting is expected to focus on a January 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.

"President Obama has accepted the (U.N.) Secretary-General's invitation to the high-level meeting on Sudan," Rice told reporters on a conference call. She added that she had conveyed Obama's acceptance to Secretary-General Ban Ki-moon on Wednesday morning.

"The president sees this meeting on the 24th as a very important vehicle for focusing international attention on ... (the referendum) as Sudan approaches really the last critical 100 days before that vote takes place," Rice said.

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 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 the referendum.

(Reporting by Patrick Worsnip; Editing by Eric Beech)