Russia, the world's largest supplier of enriched uranium, said on Friday that it had imposed temporary restrictions on the export of enriched uranium to the United States, a symbolic tit-for-tat move after the U.S. banned Russian uranium imports. Russia holds about 44% of the world's uranium enrichment capacity and about 35% of U.S. nuclear fuel imports used to come from Russia, according to the U.S. office of nuclear energy.