Juba, South Sudan (CNN) -- U.S. President Barack Obama said Thursday that South Sudan "stands at the precipice," as deadly clashes spread in Africa's newest nation. He spoke out the same day government officials said rebels have taken the town of Bor. The town, which lies about 200 kilometers (125 miles) north of the capital, Juba, was under heavy shelling, said Col. Philip Aguer, an army spokesman. Bor Mayor Mhial Majak Mhial said the town was under rebel control and heavy artillery was in use. "In 2011, millions of South Sudanese voted to forge a new nation, founded on the promise of a more peaceful and prosperous future for all of South Sudan's people," Obama said in a statement. "In recent years, against great odds, South Sudan has made great progress toward breaking the cycle of violence that characterized much of its history.
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NEW: President Barack Obama warns fighting could push South Sudan back into its past
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NEW: Unknown assailants attack a U.N. base in Africa's newest nation
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Rebels have taken the town of Bor, government officials say
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Human Rights Watch says it has reports of people being targeted for their ethnicity
Should these reports prove true, those responsible must be held accountable for their crimes," read a @placeholder statement.