(Adds U.S. Trade Representative spokesman's comment, paragraph 7-8)
MOSCOW, June 20 (Reuters) - Russia has filed a complaint with the World Trade Organisation against the United States for imposing sanctions over the Ukraine crisis, Prime Minister Dmitry Medvedev said on Friday.
Moscow had already told Washington it considered the sanctions, imposed in response to Russia's annexation of Ukraine's Crimea and its involvement in the Ukrainian crisis, to be illegal under WTO rules.
"When the United States imposed sanctions against Russia, which had a negative impact on our foreign trade, we decided to challenge these sanctions in the World Trade Organisation," Medvedev told a law conference in Russia's northern city of St Petersburg.
"We've sent a communique to the World Trade Organisation."
Washington and the European Union have imposed sanctions on several Russian and Ukrainian individuals, but Washington has also targeted a number of Russian firms and banks it says are linked to President Vladimir Putin or his close associates.
In particular, it accuses Russia of backing armed separatists fighting the government in eastern Ukraine.
A spokesman for the U.S. Trade Representative said the United States took its obligations under the WTO very seriously.
"Prior to instituting the sanctions against the Russian Federation, the United States carefully considered their consistency with WTO rules," he said.
But Medvedev said the sanctions violated WTO rules, arguing that banning service providers from another country constituted an infringement of the 'most favoured nation' status that WTO members accord each other.
Although it joined the WTO less than two years ago, Russia has already become embroiled in trade disputes with the European Union and Japan. A flurry of Russian threats and warnings suggest that more cases could soon follow.
He said challenging the United States may not be easy.
"The U.S. has both doctrinal and practical authority in the World Trade Organisation," he said. "The state is a leader in the raising of trade disputes with the WTO."
He also suggested that disputes between Russian and Ukraine companies could be solved in an arbitration court.
National security can be used as an argument to claim exemptions from WTO rules. The United States invoked national security as a member of the WTO's predecessor organisation to justify its economic embargo on Cuba. (Reporting by Lidia Kelly; Additional reporting by Krista Hughes in Washington; Editing by Tom Heneghan and Grant McCool)