Speaking during an official visit to El Salvador, one of the dwindling number of states that has diplomatic ties with the island, Taiwan Vice-President Annette Lu said El Salvador's President Antonia Saca had promised support for Taipei.
"A law of the People's Republic of China has no validity on Taiwanese territory and this one is in clear violation of the UN Charter," Lu said on Wednesday.
"This is of international concern because it violates an international treaty."
China's National People's Congress passed the law on Monday, sparking Taiwanese anger and US opposition.
"As a permanent member of the UN Security Council, China must respect the principle that international conflicts should be settled peacefully," said the Taiwanese vice-president.
El Salvador's Vice-President Ana Vilma Escobar also expressed support for Taiwan during a press conference with Lu.
Lu will end a four day trip to El Salvador on Thursday and move on to Guatamala.