Louisiana U.S. Senator Mary Landrieu's campaign has paid back over $33,000 to the federal government for political travel wrongly billed to her Senate office over a 12-year period, her campaign said on Friday.

The disclosure comes as Landrieu seeks re-election in a hard-fought race for a seat that is a top target of Republicans hoping to retake the Senate.

"The review I ordered last month found these mistakes stemming from sloppy book keeping," Landrieu said in a statement. "I take full responsibility. They should have never happened, and I apologize for this."

Landrieu, who promised a review last month after reports surfaced of improper billing of her campaign flights, said she has reported the errors to the Senate Ethics Committee and has paid back the U.S. Treasury with campaign funds.

In 2002, federal election and Senate travel rules changed to require that most trips where both official and campaign business occur be prorated between the two accounts. Previous to that, trip expenses could only be paid by one account.

A spokesman for U.S. Representative Bill Cassidy, Landrieu's most prominent Republican challenger, criticized her campaign for releasing records dating only to 2002, and not to 1996, when Landrieu first won her seat.

"No matter how Senator Landrieu spins it, she billed taxpayers for dozens and dozens of private jet flights since the turn of this century," John Cummins, a Cassidy spokesman, said in a statement.