Washington/San Francisco: Intel faces a formal investigation by the US Federal Trade Commission (FTC) after fighting similar antitrust probes in Europe and Asia, the company said.

Smaller rival Advanced Micro Devices has long accused Intel of abusing its dominance of the $280 billion chip market and filed its own lawsuit against Intel in 2005. Intel's microprocessors power more than 80 per cent of the world's PCs.

An FTC spokeswoman confirmed the case was upgraded to a formal probe, but declined to say more.

Intel General Counsel Bruce Sewell said the company already gave the FTC hundreds of thousands of documents in the agency's two-year-old informal probe and would continue to cooperate.

"We don't believe that there's been abusive behav-iour or illegal behaviour," he said. "The record here is so strong in our favour."

The FTC investigation was a second blow last week for Intel, whose stock was down 3.35 per cent at $23.07 in afternoon trading on the Nasdaq.

On Thursday, South Korea ruled Intel abused its dominant position in the local market and imposed a fine of $25.6 million. Intel said it would appeal.

A former FTC staffer said the resignation of chairman Deborah Platt Majoras in March set the stage for the tougher probe.