Samsung Electronics Co plans to add Apple Inc's iPhone 5 to existing patent lawsuits against its U.S. rival, stepping up its legal challenge to Apple as the two smartphone leaders seek to assert rights to key technologies.

The fresh legal step by the South Korean group comes as Apple booked orders for over 2 million iPhone 5s in the first 24 hours of its availability. The product is due to hit stores on Friday.

Samsung and Apple are locked in patent battle in 10 countries and the stakes are high as the two vie for top spot in the booming smartphone market.

Both companies are also raising marketing spending to promote their latest products ahead of the crucial year-end holiday season.

"Samsung anticipates that it will file, in the near future, a motion to amend its infringement contentions to add the iPhone 5 as an accused product," the company said in a U.S. court filing.

"Based on information currently available, Samsung expects that the iPhone 5 will infringe the asserted Samsung patents-in-suit in the same way as the other accused iPhone models."

Samsung said in a separate statement on Thursday: "Apple continues to take aggressive legal measures that will limit market competition.

"Under these circumstances, we have little choice but to take the steps necessary to protect our innovations and intellectual property rights."

Apple Korea reiterated its position that it was the victim of copying, not vice versa.

"At Apple, we value originality and innovation ... We make these products to delight our customers, not for our competitors to flagrantly copy."

Apple scored a legal victory over Samsung in late August when a U.S. jury found the Korean firm had copied critical features of the iPhone and awarded the U.S. firm $1.05 billion in damages.

The jury also found that Apple did not infringe any of Samsung's asserted patents. Samsung has since vowed it would continue to fight.

Apple's victory was also a blow to Google Inc, whose Android software powers the Samsung products that were found to infringe Apple patents.

Samsung, the biggest Android phone maker, received a second U.S. legal setback last week when a judge at the International Trade Commission said in a preliminary ruling that Apple did not violate patents owned by Samsung.

The Korean firm was the world's top smartphone maker in the second quarter of 2012, shipping more than 50 million phones, nearly double the Apple's 26 million iPhone shipments.