A U.S. appeals court wrestled on Friday with a request by Apple Inc for a permanent injunction on sales of some phones made by its archrival, Samsung Electronics Co Ltd, in a case that could have a deep impact on patent litigation.

Friday's hearing came hours ahead of an expected ruling by the International Trade Commission in a separate case on whether certain mobile devices made by South Korea's Samsung violate Apple patents. The ruling is expected around 5 p.m. EDT (2100 GMT).

Apple is appealing a lower court ruling that rejected the iPhone maker's request that some older-model Samsung phones be permanently banned because they violate an Apple patent.

At stake is whether judges can permanently ban the sale of a complex device like a smartphone if a court finds that it violates a patent that covers just one of the device's hundreds of features. Samsung and other smartphone makers say a decision in favor of Apple could cripple the market.

At an hour-long hearing in Washington, U.S. Federal Circuit Court of Appeals Judge William Bryson questioned whether Apple wanted to use the appeal to set a precedent that would allow it to seek sales bans for newer Samsung phones in an expedited fashion.

"Is that really what we're dealing with?" Bryson asked.

The current case has dragged on for two years.

Apple lawyer William Lee acknowledged that if the iPhone maker's injunction request was granted, the company would seek a so-called contempt proceeding to go after newer Samsung phones.

The case is one of many between the two phone makers as they accuse each other of patent violations in a bid to dominate a fast-growing market for mobile devices.

Samsung's popular Galaxy smartphones and tablets run on Google's Android operating system, which Apple's late co-founder, Steve Jobs, once denounced as a "stolen product." Apple says Samsung has infringed its patents and caused financial losses. Samsung denies it copied Apple's patented features.

The three-judge panel on Friday weighed whether the lower court was right in requiring Apple, when requesting a sales ban, to prove a "causal nexus" connecting a dip in iPhone or iPad sales to Samsung's alleged specific patent infringements.

"I can't imagine that you wouldn't have some nexus requirement, regardless of how you define it, in every case of injunction," Judge Kathleen O'Malley said.

But O'Malley also questioned whether Apple even could prove that one feature of a complex product is the single driver of consumer demand, as demanded by the lower court. She said that if Apple proves one patented feature - such as "pinch-to-zoom" -to be the most important driver of consumer demand, it would automatically disprove the same about other patents.

LONG LEGAL HISTORY

Samsung has said that all of the phones involved in the ongoing patent litigation are either no longer in the market or include design workarounds. Apple says the newer phones continue to rely on the same features at stake in this case.

Apple last year won a big monetary verdict against Samsung from a Northern California jury in the patent infringement case but the iPhone maker has been less successful in convincing judges to order injunctions.

The Federal Circuit last year rejected Apple's request for a pretrial sales ban against Samsung's Galaxy Nexus phone. With that, it raised the bar for injunctions on device sales based on narrow patents for the devices' features.

It also put Samsung in a much stronger position by allowing its products to remain on store shelves while it fights the global patent battle.

U.S. District Judge Lucy Koh last December used that Federal Circuit's new, tough pretrial injunction standard when she rejected Apple's bid for a permanent post-trial injunction on several Samsung phones. She also slashed Apple's $1.05 billion jury verdict.

In its appeal, Apple says Koh's ruling would "create a bright-line rule" and argues she should not have used that tougher standard.

Google Inc, HTC Corp and others have chimed in on Samsung's behalf, arguing that an Apple victory would return patent law to the days "when injunctive relief automatically followed a finding of infringement."

The appeals court is expected to issue a written opinion but did not indicate a timeline.

The case is Apple Inc v. Samsung Electronics Co Ltd, Federal U.S. Circuit Court of Appeals, No. 13-1129.

(Editing by Ros Krasny, Vicki Allen, Bernadette Baum and Matthew Lewis)