TAIPEI (Reuters) - Taiwan President Tsai Ing-wen pledged deeper ties with the United States at a dinner for a visiting senior State Department official, on the same day China sent 18 fighter jets to harry the island to express its anger at the trip. China had earlier on Friday announced combat drills and denounced what it called collusion between the island, which it claims as part of its territory, and the United States. Taiwan scrambled its air force in response. U.S. Undersecretary for Economic Affairs Keith Krach arrived in Taipei on Thursday for a three-day visit, the most senior State Department official to come to Taiwan in four decades. Hosting a dinner for Krach and his team at her official residence, Tsai thanked him for coming to attend Saturday’s memorial service for late President Lee Teng-hui, the father of Taiwan’s democracy, the presidential office said in a statement late Friday. “I hope that Taiwan and the United States continue to work together to promote peace, stability, prosperity, and development in the Indo-Pacific, positively impacting the region,” it cited Tsai as saying. Taiwan-U.S. relations have made substantive progress in recent years, “with increasingly close interaction”, she added. “I hope that we are able to engage on even more issues and expand our bilateral cooperation, enhancing our relationship as trusted partners and building a solid foundation for the future.” Taiwan is committed to deepening economic cooperation with the United States, Tsai said. The statement made no mention of the Chinese drills. Taiwan has been angling to sign a free trade deal with the United States, and has been keen to show Washington the economic benefits Taiwan can bring it. In May, Taiwan Semiconductor Manufacturing Co Ltd, the world’s biggest contract chipmaker, said it plans to build a $12 billion factory in Arizona in an apparent win for the Trump administration’s efforts to wrestle global tech supply chains back from China. TSMC’s retired founder Morris Chang attended Tsai’s dinner with Krach, and appeared prominently in pictures of the event put out by the presidential office.