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-in, which took place at the White House, was a precautionary measure to remove any possible doubt about his legitimacy.

"We believe that the oath of office was administered effectively and that the president was sworn in appropriately yesterday," Craig said in a statement.

"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."

Tuesday's noon ceremony on Washington's National Mall was witnessed by an unprecedented audience of well more than 1 million people as Obama became the first African-American president of the United States. Roberts' mistake caused Obama to briefly stumble on his own words during the historic moment.

Wednesday's repeat performance took place at 7:35 pm (0035 Thursday GMT) in the White House map room with only a few people present.

"We decided it was so much fun," Obama, 47, joked while sitting on a couch.

Roberts, 53, slipped on his black judicial robe before asking, "Are you ready to take the oath?"

Obama, who voted as a US senator against Roberts' 2005 nomination to the Supreme Court, replied: "I am, and we're going to do it very slowly."

It took the two men 25 seconds to repeat the process, including the exact wording of the presidential oath as stated in article two of the Constitution: "I do solemnly swear that I will faithfully execute the Office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States."

Roberts smiled and said: "Congratulations, again."

Obama thanked him, and the handful of witnesses clapped.

The chief justice's error was the subject of a rather insensitive joke by Vice President Joe Biden during the swearing in of Obama's senior White House staff on Wednesday. "My memory is not as good as Justice Roberts," Biden quipped, drawing a stone-faced look from Obama. (dpa)