Production lines at Ssangyong Motor were partially halted on Friday after some unionised workers walked off their jobs for several hours to protest the planned sale of the vehicle maker, a union spokesman told Dow Jones Newswires.

The spokesman reportedly said some 4,500 union workers out of a total 7,000 employees have stopped work since Thursday for two hours during the day while an additional 800 union members are expected to walk out from the plant sometime next week.

However, Dow Jones said that a Ssangyong spokesman on Friday denied that the company's production lines have been affected by the walkout.

The report said that the union has been protesting creditors' plan to sell a majority stake in the company, saying that creditors are "unjustly seeking to sell Ssangyong Motor without considering the automaker's independent survival."

According to Dow Jones, Ssangyong's main creditor, Chohung Bank, had named Samil Accounting as lead manager for an international auction of the creditors' 55.4% stake in the carmaker earlier in November.

After debt-for-equity swaps in 2000 and 2002, creditors of Ssanyong Motor currently have a combined 55.4% stake, or 67 million shares, in the troubled car maker, Dow Jones said.

Ssangyong specialises in SUVs powered by Mercedes-Benz  engines it makes under licence.

According to the Dow Jones report, local media on Thursday reported that at least four overseas carmakers had submitted proposals to buy a controlling stake in the company, South Korea's fourth largest automaker. The deadline for the bid proposals was Thursday and creditors will then decide on a preferred negotiator, although a specific timeframe hasn't been given, the report added.

Dow Jones said that General Motors, Renault, and China's Nanxing Group and Shanghai Automotive Industry Corp. have reportedly submitted bids for the stake.