Taiwan's official Central News Agency (CNA) reported on Friday that a Chinese PLA guided-missile destroyer was detected in the danger zone of Taiwan's missile tests several hours before the drills began. 

In July, state-owned weapons developer and manufacturer, National Chung Shan Institute of Science and Technology (NCSIST), said that it would conduct missile tests on August 18-19 and August 25-26 from the Jiupeng base in Pingdong province in the south of the island.

The projectiles would be fired to the east and northeast of the coastal waters of Taitung county. The NCSIST flagged a 200-kilometer strip extending from the coast of the province as a danger zone but did not specify any altitude limit. Although the institute did not disclose what rockets were supposed to be tested, there were speculations that Taiwan would test the Hsiung Sheng cruise missiles, the CNA reported.

Taiwan successfully launched missiles from the Jiupeng base and from the coastal town of Chenggong in Taitung county on 8:40 p.m. local time (12:35 GMT) on Thursday. The projectiles were discharged above the waters between the Green and the Orchid islands.

A Navy source told the media on condition of anonymity that a guided-missile destroyer of the Chinese People's Liberation Army was spotted 61 kilometers to the northeast of Green island, within the danger zone, just hours before the tests began.

The NCSIST is one of the two major defense contractors of the self-governed island, actively involved in the development, manufacturing, and sale of defense systems and weapons.

Read more: China sanctions 7 pro-separatist Taiwanese officials

In early August, US House Speaker Nancy Pelosi visited Taipei while on an Asia tour despite warnings from China, which considers Taiwan part of its territory and opposes any direct official foreign contacts with the island. She was the first highest-ranking US official to visit Taiwan in 25 years.

The visit triggered a new round of tensions in the Taiwan Strait. In addition to the launch of military exercises, Beijing had imposed sanctions against two Taiwanese foundations for separatist activities, suspended the export of natural sand to the island and the import of citrus fruits, as well as some types of fish products from Taiwan.

The arrival of five other US delegates to Taiwan has reignited tensions even further, with China vowing zero tolerance for "separatist activities" in Taiwan and reaffirming that it would take the self-ruled island by force if necessary.

Read more: China conducts fresh drills around Taiwan as US lawmakers visit