Election officials said Monday that more than 98 percent of ballots in the Jan. 9 vote were in favor of independence, meaning Southern Sudan will become the world's newest country in July.
"I congratulate the people of Southern Sudan for a successful and inspiring referendum in which an overwhelming majority of voters chose independence," President Barack Obama said in a statement. "I am therefore pleased to announce the intention of the United States to formally recognize Southern Sudan as a sovereign, independent state in July 2011."
Obama called it "another step forward in Africa's long journey toward justice and democracy."
Secretary of State Hillary Rodham Clinton commended the Sudan government for accepting the outcome.
Clinton said in a statement that the designation will be lifted if Sudan does not support terrorism for the preceding six months and provides assurance it will not do so in future. It must also fully implement a 2005 peace agreement that ended a two-decade civil war between the north and south that killed more than 2 million people.
Clinton urged both northern and southern leaders to continue to work together toward full implementation of the peace agreement and post-referendum arrangements, to ensure they become two "viable states living alongside each other in peace."
The mainly Christian south and mainly Muslim north must still negotiate citizenship rights, oil rights and border demarcation. Virtually all of southern Sudan's budget comes from oil revenue, and the north wants to maintain fuel supplies from the south.
Sudan's President Omar al-Bashir, who has been indicted for war crimes in the western Sudan region of Darfur, on Monday backed the vote results and said he wanted to be the first to congratulate the south on their new state.
Obama demanded an end to attacks on civilians in Darfur. He said the U.S. supported the aspirations of all Sudanese, and would work with the governments of Sudan and Southern Sudan for a smooth and peaceful transition to independence.
"For those who meet all of their obligations, there is a path to greater prosperity and normal relations with the United States, including examining Sudan's designation as a State Sponsor of Terrorism," he said.
State Department spokesman P.J. Crowley told a news conference in Washington that the government of Sudan has made clear that it wants normal relations with the United States.
Sudan has been on the U.S. list of states that sponsor terrorism since 1993.
(This version corrects "overwhelmingly" to "overwhelming" in 3rd paragraph.)