General Motors Corp., the world's largest automaker, has said US financial regulators are investigating its pension accounting and dealings with its bankrupt former parts unit Delphi Corp.

News of the probe sent GM shares plunging by 5.5 percent to $27.57 on Wall Street. The Securities and Exchanges Commission has issued subpoenas to GM, and the company is cooperating with the investigation, GM said on its website.

The Detroit-based automaker's finance unit, General Motors Acceptance Corp., is also being investigated in a separate industry-wide probe of the insurance industry, GM said Thursday.

The latest woes come as GM struggles to lower the burden of high wages, pension costs and other retiree benefits secured over the years by the powerful United Auto Workers Union. Delphi's bankruptcy could force GM also to take over some of its former subsidiary's worker benefits.

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