SAN FRANCISCO (Reuters) - Apple Inc AAPL.O and Intel Corp INTC.O on Wednesday filed an antitrust lawsuit against Fortress Investment Group, alleging the SoftBank Group Corp 9984.T unit stockpiled patents to hold up tech firms with lawsuits demanding as much as $5.1 billion.

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The lawsuit follows an earlier case that Intel filed against Fortress in October. Intel withdrew that lawsuit and on Wednesday filed a new version in the U.S. District Court for the Northern District of California with Apple joining as a plaintiff.

Intel and Apple allege that Fortress and firms it either owned or whose patent portfolios it effectively controlled - and which do not make any technology products - stockpiled patents for the primary purpose of suing technology companies and did so in a manner that violated U.S. antitrust laws.

“Apple has suffered economic harm in the form of litigation costs and diversion of resources away from innovation to respond to these entities’ serial nuisance suits,” Apple wrote in the complaint.

SoftBank declined to comment.

“We do not comment on the specifics of pending litigation, but we remain confident in our business practices and our legal position,” Gordon Runté, managing director at Fortress, told Reuters in an email. “We view this lawsuit as meritless and look forward to a court’s decision on the matter.”

In the filing on Wednesday, Apple said firms connected to Fortress had filed at least 25 lawsuits against the iPhone maker demanding $2.6 billion to $5.1 billion in damages. One of the suits, Apple wrote in its filing, alleged that Apple violated a step-counting patent in its devices with health-tracking apps.

Two Fortress-connected firms, Uniloc USA and Uniloc Luxembourg, “have disclosed that they believe they are entitled to damages of between $1.41 and $2.75 per Apple product, for total damages in the range of $375 to $732 million,” Apple wrote in its complaint.

"The apparent precision of the per-unit damages request is a facade; Uniloc USA and Uniloc Luxembourg simply adopted the amounts that Apple sought from Samsung in litigation for Apple's patents," Apple wrote, referring to South Korea's Samsung Electronics Co Ltd 005930.KS.

Intel said it could not comment beyond the court filing. Apple, which invested in SoftBank’s first Vision Fund and in July was listed among organizations pledging to contribute to a second fund, did not immediately respond to a request for comment.