About 2,700 police backed by water cannon and helicopters moved into the main plant of supplier Yoosung Enterprise on Tuesday to evict 540 strikers occupying the premises.

Yoosung makes piston rings, cylinder liners and camshafts for the country's automakers, some of which have had to cut production since the dispute began over pay and working conditions.

Hyundai said it would try to resume normal production as soon as possible, although this depended on when Yoosung got back to normal.

"Production was partially normalised today at our diesel engine lines," said a spokesman at the company's main plant in the southeastern city of Ulsan.

Hyundai and its affiliate Kia Motors are the world's fifth-largest automaker by sales. They source piston rings for around 70 percent of their vehicles from Yoosung, whose main plant is at Asan, 80 kilometres (48 miles) south of Seoul.

The strike had forced Hyundai to stop production at two diesel engine lines.

Police moved in on Yoosung after declaring the work stoppage illegal because strikers barred non-strikers from the plant and occupied the premises.

Police were on guard at the plant Wednesday to help non-union members normalise operations, but it was still unclear when production would resume.

Officers Tuesday detained some 500 of the 540 employees occupying the factory.

"Some 400 workers have now been released while 100 are still in detention for questioning," a police spokesman in Asan told AFP.

Copyright AFP (Agence France-Presse), 2011