WASHINGTON (Reuters) - President Barack Obama said on Friday that Mitt Romney should answer questions about whether he worked for Bain Capital longer than previously described, escalating the attack on his Republican rival's tenure as a private equity executive.

Romney has said he left the firm in 1999, when he was tapped to lead the Salt Lake City Olympics. But the Boston Globe reported on Thursday that public records indicate he was still registered as a top official at Bain for three more years.

Timing matters because Romney has said he left Bain in 1999 and thus was not responsible for bankruptcies and layoffs at Bain-owned businesses after that time, which Obama's re-election campaign have used to question the Republican's track record.

"Ultimately Mr. Romney, I think, is going to have to answer those questions, because if he aspires to being president one of the things you learn is, you are ultimately responsible for the conduct of your operations," Obama told ABC television affiliate WJLA in an interview.

Citing Securities and Exchange Commission documents, the newspaper said Romney remained Bain's "sole stockholder, chairman of the board, chief executive officer and president" until 2002, when he and the firm finalized a severance deal.

"My understanding is that Mr. Romney attested to the SEC, multiple times, that he was the chairman, CEO and president of Bain Capital and I think most Americans figure if you are the chairman, CEO and president of a company that you are responsible for what that company does," Obama said.

Romney says he gave up day-to-day control of Bain in 1999. He has continued to earn money from his Bain investments since the severance deal expired; a disclosure form filed recently revealed roughly $2 million in new Bain income.

(Reporting By Alister Bull; Editing by Eric Walsh)