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STOCKHOLM (Reuters) - Volvo Cars is doubling investment in its first U.S. plant under construction to add a second production line at a total cost of $1 billion, a source with knowledge of the plans said, as the carmaker heads for a fourth straight year of record sales.

The move to raise capacity even before completion of the plant in Charleston, South Carolina will ultimately add 2,500 workers to the 2,000 already being recruited by the Swedish carmaker ahead of its 2018 opening, the source told Reuters.

A Volvo spokeswoman declined to comment. The increased investment was first reported by the Charleston-based Post and Courier newspaper.

The Charleston plant expansion is a vote of confidence in Volvo’s sales momentum, buoyed by a revamped lineup led by the flagship XC90 SUV, even as U.S. auto market growth tails off.

Under the ownership of Chinese carmaker Geely's 0175.HK unlisted parent company, Zhejiang Geely Holding Group, Volvo has recovered from a sales slump in its final years under previous owner Ford F.N and is now challenging larger premium rivals such as BMW BMWG.DE and Mercedes-Benz DAIGn.DE.

The company, one of Sweden’s largest by revenue, has blamed supply shortages for a 7 percent dip in U.S. deliveries so far this year and vowed to achieve year-on-year sales growth for 2017 as a whole, along with a new global record.