BEIJING, June 22 (Reuters) - China"s longest-serving ambassador to Washington, Cui Tiankai, said on Tuesday he will be leaving his post after eight years, amid strained relations between the world"s two largest economies. Cui, known for his more congenial, diplomatic manner, is expected to be replaced by Qin Gang, a trusted aide of President Xi Jinping known for his sharp retorts to criticism of China. China"s foreign ministry declined to comment on Tuesday on who would succeed Cui, but sources with knowledge of the matter have told Reuters that Qin is expected to take over. Qin, 55, a foreign ministry vice minister, had served Xi as his chief protocol officer between 2014 to 2018. During two stints as foreign ministry spokesman from 2006-2010 and 2011-2014, Qin was known for striking an assertive posture, often making sharp comments in defence of China. Asked at a press conference in February about China"s so-called "wolf warrior" diplomacy - an assertive and often abrasive style adopted by many Chinese diplomats in recent years - Qin defended China"s right to reject the "baseless smears." During the previous U.S. administration under President Donald Trump, U.S. relations with China plunged to their lowest point since the establishment of diplomatic ties in 1979, with the countries clashing over issues ranging from trade and technology to Hong Kong, Taiwan and Xinjiang, and the South China Sea. Making his departure public in a farewell letter posted on the embassy"s website, Ambassador Cui, who is 68 and past the retirement age of 65 for senior Chinese diplomats, wrote: "I will forever treasure the deep friendship forged with the Chinese nationals here during my posting in the United States."