Taiwan vowed on Tuesday to fight China's "increasingly out of control" behavior after Taipei lost another ally to Beijing when El Salvador became the third country to switch allegiances to China this year.

Taiwan now has formal relations with only 17 countries worldwide, many of them small, less developed nations in Central America and the Pacific, including Belize and Nauru.

Speaking in Taipei, President Tsai Ing-wen said Taiwan would not bow to pressure, describing El Salvador's decision as further evidence of China's efforts to squeeze the island, which have included regular Chinese bomber patrols around Taiwan.