Washington  - US President Barack Obama was forced to retake the oath of office late Wednesday, after Chief Justice John Roberts erred on one word during Tuesday's inaugural ceremony on the Capitol steps, the White House said.

Greg Craig, the administration's top lawyer, said the second swearing, which took place at the White House, was a precautionary measure.

"We believe that the oath of office was administered effectively and that the president was sworn in appropriately yesterday. But the oath appears in the Constitution itself. And out of an abundance of caution, because there was one word out of sequence, Chief Justice Roberts administered the oath a second time," Craig said in a statement.

Tuesday's ceremony took place on Washington's National Mall an unprecedented audience of well over 1 million people as Obama became the first African-American president of the United States. (dpa)