A US Navy ship has sailed close to artificial islands built by China in the disputed waters of the South China Sea, US defence officials have said, reports BBC.Guided-missile destroyer USS Lassen breached the 12-nautical mile zone China claims around Subi and Mischief reefs in the Spratly archipelago.The freedom of navigation operation represents a serious challenge to China`s territorial claims.The foreign ministry in Beijing condemned it as "illegal".In a statement, ministry spokesman Lu Kang described the operation as a "threat to China`s sovereignty", adding that Beijing would "resolutely respond to any country`s deliberately provocative actions".US Defence Department spokesman Commander Bill Urban said earlier that "the United States is conducting routine operations in the South China Sea in accordance with international law".DredgingChina claims most of the South and East China seas. Other countries in South East Asia have competing claims for the Spratly Islands, Paracel Islands and Scarborough Shoal, which are thought to have resource-rich waters around them.The reefs, which were submerged, were turned into islands by China by a massive dredging project which began in late 2013.China says this work is legal and in a meeting with US President Barack Obama last month in Washington, President Xi Jinping said China had "no intention to militarise" the islands.But Washington believes Beijing is constructing military facilities, designed to reinforce its disputed claim to most of the region - a major shipping zone.