The Daily Mail reported Tuesday that the money is owed to the public relations firm of Mark Penn, who resigned from the New York senator's campaign amid a flap over meeting with the Colombian government to discuss advancing a trade deal Clinton opposes.

The money Penn is owed accounts for more than a quarter of the campaign's $8.7 million in outstanding debt, according to finance reports filed last month.

The Clinton campaign pays Penn through his business -- Penn, Schoen & Berland Associates. The campaign has paid the firm at least $10 million since the presidential race began last year, the newspaper reported.

"Certainly, the unpaid bills create a continuing relationship," said Massie Ritsch, spokesman for the Center for Responsive Politics, a campaign finance watchdog group.

"The amount of money that they have billed would not appear to be unusual for a consultant working on media polling and other aspects of a major presidential campaign," he added.