"In Wednesday's edition of the Frankfurter Allgemeine Zeitung, Volkswagen CEO Martin Winterkorn commented on rumors regarding Opel, which continues a regrettable pattern of fanning speculation as Opel makes solid progress in its restructuring, in generating improved operating results," GM said in a statement.
Speculation about Opel's future began in early June with reports by German media the European unit could be sold and possible buyers included Chinese automakers or Volkswagen.
Opel Chairman Nick Reilly said on June 30 GM was "very satisfied" with Opel's progress, but failed to quash rumors GM was mulling a sale, saying the company does not comment on speculation.
However, Opel remains a high-cost player in a low-growth region, in a segment dogged by cutthroat competition, leading to speculation of a sale or labor concessions by its workers.
On Wednesday, GM again did not specifically address whether Opel was for sale or not, saying it has a long-standing policy of not commenting on speculation.
"Opel has been part of the GM family since 1928 and remains important to the company," GM said on Wednesday. "GM is pleased with Opel's solid progress over the last year in turning around its business, and the company continues to invest in outstanding products for the European market."
German government leaders and Opel union officials have been frustrated with GM's refusal to directly address the rumors.
GM dropped plans to sell Opel in 2009 after months of negotiations and embarked on a restructuring to get the unit, which lost $1.6 billion last year, back on track.
Winterkorn told the German newspaper he did not think South Korean automaker Hyundai Motor Co (005380.KS) would be interested in buying Opel if it was sold, but Chinese automakers would love to gain access to Opel's technical center and distribution network.
Winterkorn added it might not be wise to give up 5,000 engineers at a time when everybody is looking to hire such people, according to the newspaper.
VW declined to comment and GM would not discuss the issue beyond its statement.
People familiar with GM's thinking previously told Reuters GM CEO Daniel Akerson was reviewing options for Opel, a unit he was in favor of selling in the past and with which he remained frustrated.
At GM's June annual meeting, Akerson was clear where Opel stood in GM's portfolio, calling it a regional brand that would support the two global brands of Chevrolet and Cadillac.
In it Wednesday statement, GM questioned the timing of Winterkorn's interview, saying it came at the same time as positive reviews for the Opel Ampera plug-in hybrid vehicle. The Ampera is the European version of the Chevrolet Volt.
In May, GM boosted production plans for the Volt and Ampera for 2011 and 2012, citing strong demand.
Reporting by Ben Klayman in Detroit; editing by Robert MacMillan and Andre Grenon