HELSINKI (AFP) - Nokia, the world's biggest mobile phone maker, said on Tuesday it had asked the US International Trade Commission (ITC) to open an inquiry into alleged infringement of Nokia patents by Apple.

Nokia has already filed a legal case against Apple, which has countersued the Finnish giant, in a fierce battle between the two mobile world titans that has seen a sharp drop in Nokia sales and a rise in Apple's iconic iPhone.

Nokia's complaint focuses on seven Nokia patents and alleges that "Apple infringes Nokia patents in virtually all of its mobile phones, portable music players and computers," Nokia said in a statement.

"These patented technologies are important to Nokia's success as they allow better user experience, lower manufacturing costs, smaller size and longer battery life for Nokia products," the statement said.

Nokia patents manager Paul Melin said: "Nokia has been the leading developer of many key technologies in small electronic devices. This action is about protecting the results of such pioneering development."

Nokia filed its legal complaint against Apple in October in a federal court in the eastern state of Delaware.

Nokia accused Apple of infringing 10 patents covering "wireless data, speech coding, security and encryption and are infringed by all Apple iPhone models shipped since the iPhone was introduced in 2007."

Apple hit back in December, alleging Nokia infringing Apple patents.