The Port of Los Angeles is asking for a $1.57 million refund from six freight haulers who did not meet their end of a financial deal that paid for fuel-efficient "clean trucks," officials said Monday.

The companies are accused of failing to make at least 150 trips to Los Angeles with each of the trucks they purchased with funds received through the port's Clean Trucks Program incentive plan. Port officials will issue final bills at the end of this month, seeking "immediate reimbursement."

"There are a handful of companies that have yet to fulfill their obligation and the final billing process seeks to recover the funds owed to the port," said Phillip Sanfield, a spokesman for the Port of Los Angeles.

About six weeks before the Clean Trucks Program went into effect in 2008, port officials touted a new partnership with Phoenix-based Swift Transportation, which received more than $11.8 million to purchase 591 trucks.

Now, Swift must return more than $1.46 million because 428 of those port-funded trucks did not make at least 150 annual trips to Los Angeles, officials said.

"We value our relationship with the Port of Los Angeles and we're proud of the role Swift played in improving air quality at the port," said Dave Berry, vice president of Swift Transportation.

Berry declined to comment on the amount of money his company owes the port.

The Clean Trucks Program incentive provided haulers with $20,000 for every truck that met 2007 federal emission standards, with the goal to quickly replace older, pollution-spewing models.

In the end, about $44 million was issued to 56 freight haulers to purchase nearly 2,200 trucks, according to a port report.

The deal initially required each truck to make at least 300 trips to the port to ensure a return on the investment. A drastic decline in cargo containers coming through the port prompted the harbor commission last year to slash the threshold in half.

"We did not want to penalize the trucking companies for failing to meet the requirements due to an economic downturn," Sanfield said.

Along with Swift, five other companies are being asked to return some of the cash incentives provided by the port.

Meyer Trucking Inc. owes $80,000 because 26 of its 30 port-funded trucks did not make the required number of trips to Los Angeles Harbor, officials said.

Central Cal Transportation owes $16,000 for four trucks, while LMD Integrated Logistic Services owes $3,000 for one truck. Additionally, Knight Transportation and Pacer Cartage each owe $4,000 for one truck.

Failing to repay the funds could be considered a default and prompt port officials to suspend or cancel their agreements with the delinquent trucking companies, Sanfield said. Such a move would bar them from hauling shipments in and out of Los Angeles.

The neighboring Port of Long Beach issued $30 million to independent truck drivers and small freight hauling companies, all of whom met a requirement to make at least 300 trips to the port.