General Motors said on Monday it had yet to decide whether to take part in an equity increase at Italy's Fiat, in which it holds a 20% stake, Reuters said.

"What we've said so far is we have not made a decision to participate," chief executive Rick Wagoner told a media conference in London, according to Reuters. "That hasn't changed."

Reuters noted that Fiat SpA group, which owns 80% of Fiat Auto, revealed in February a five-billion-euro ($US5.79 billion) recapitalisation for its sputtering car unit.

According to the news agency, Fiat could compel GM to buy its stake in Fiat Auto from 2004 under a "put" option the two companies agreed in 2000 as part of a wider industrial alliance, but Fiat said in March it hoped to avoid ever forcing GM to buy the rest of the car unit.