Other offers, besides Magna's Russian-backed bid, are welcome, Fred Irwin, the head of the trust company managing Opel, told the German business daily Handelsblatt on Wednesday.
A delegation from BAIC (Beijing Automotive Industry Corporation) is to meet Opel managers this week and has been granted access to its accounts, as has the investment fund Ripplewood - a former bidder -the newspaper said.
"All interested parties have the same information rights," Irwin said.
He added that a letter of intent signed last month with Magna, which assembles cars and makes auto parts, was not contractually binding.
A GM spokesman told the paper, "We are in talks with several bidders. The outcome is open."
German government last month unveiled a rescue plan for Opel with General Motors, which has declared bankruptcy in the United States. Under the plan, 55 percent of Opel was to be sold to Magna but talks have become bogged down since and GM appears to be trying to put pressure on the Canadian company in order to obtain the best deal.
Ripplewood, acting through the holding company RHJ International, and BAIC, had both expressed interest in Opel before the Magna deal was announced, but were eliminated following an examination of their offers. The Italian car maker Fiat had also bid for Opel.
Until a final agreement is reached, Opel is being managed by a trust company which is financed by German public credits.