Aides to Sen. Charles Schumer (search) said his campaign was returning about $20,000 to the federal government after his Republican challenger questioned his campaign's billing for chartered planes.

Staffers for Howard Mills (search), a state assemblyman campaigning to unseat the Democratic senator, had claimed Schumer was using taxpayer money to pay for private plane flights to fund-raising and campaign events.

Aides to Mills provided their findings to The New York Times, which contacted Schumer's office for a response, the newspaper reported Wednesday.

Schumer's staff then reviewed its records and conceded that public funds had been used improperly campaigning or fundraising on 35 occasions, the Times said.

"Over the last five and a half years, the review found approximately 35 trips with accounting errors, totaling less than $20,000 which is less than 5 percent," the campaign said in a statement.

Schumer "has made full reimbursement in accordance with all procedures," the statement said.

The Times said the Mills campaign initially accused Schumer of improperly billing flights at least 65 times, but that Schumer staffers were able to provide proper documentation for many of the flights.

The newspaper said other allegations made by the Mills camp — including charges that Schumer was flying in luxury and took flights of less than 35 miles — were unfounded.

Still, Mills's campaign manager, Kevin Collins, said Schumer "needs to be held accountable."

Earlier this month, Mills aides retracted a claim that that he had passed more bills than Schumer.