Led by Hyundai Motor, South Korea's car makers increased sales 7% year-on-year in January on surging exports, but analysts warned that tough US competition and sluggish domestic sales could mar prospects for the full year, Reuters reported. South Korea may now be the world's fifth-largest producer of cars, behind the United States, Japan, Germany and France, but the country's manufacturers now face heavyweight rival General Motors leading a relentless price war in their key US export market and a strong Korean currency making their products more expensive, Reuters said.