The world will be watching Sudan's government as millions of southern Sudanese begin voting on Sunday in a historic referendum, he wrote in an editorial to be published in The New York Times, and the international community is determined that the vote is orderly, without violence.

"Today, I am repeating my offer to Sudan's leaders -- if you fulfill your obligations and choose peace, there is a path to normal relations with the United States, including the lifting of economic sanctions and beginning the process, in accordance with United States law, of removing Sudan from the list of states that sponsor terrorism," Obama wrote in the editorial, which the White House released on Saturday.

"In contrast, those who flout their international obligations will face more pressure and isolation," he wrote.

Obama said voters must be allowed to cast ballots without intimidation, all sides should refrain from provocation, election officials must not be pressured and leaders from both the north and south must work together to prevent violence.

Millions of southern Sudanese begin voting on Sunday in a historic referendum expected to see the war-ravaged south of Africa's largest country emerge as a new nation.

The Sudanese vote has been organized by the International Organization for Migration under a 2005 peace agreement. If, as expected, the vote is for independence, the south will become a new country shortly after an interim period ends on July 9.

Obama also said there can be no enduring peace in Sudan without lasting peace in the western region of Darfur, where hundreds of thousands of people have died. He said Sudan's government must also live up to its obligations in Darfur, attacks on civilians must stop and peacekeepers and aid workers must be allowed to do their work.

Obama is the first African-American president of the United States. His father was born in Kenya.