WASHINGTON (Reuters) - The sale of four sophisticated U.S.-made aerial drones to Taiwan has crossed a key hurdle in Congress and is at the last stage of approval, sources said on Monday, a deal likely to add to already strained ties with China. The $600 million deal would be the first such sale since U.S. policy on the export of sophisticated and closely guarded drone technology was loosened by the Trump administration. Reuters reported in recent weeks on the administration moving ahead with four other sales of sophisticated military equipment to Taiwan, with a total value of around $5 billion, as it ramps up pressure on China and concerns rise about Beijing’s intentions toward the island.