Taiwan said on Thursday that China sent navy ships and a large group of fighter jets towards the self-ruled island.

Taiwan’s Defense Ministry said the Chinese People’s Liberation Army sent 33 warplanes and 6 navy vessels between 6 a.m. Wednesday to 6 a.m. Thursday. The J-10 and J-16 fighter jets flew across the midline and to the southwest of Taiwan.

The Ministry said it used land-based missile systems to track the aircraft, 10 of which crossed the midline of the Taiwan Strait, an unofficial boundary that had been considered a buffer between the island and China.

On Wednesday, the Ministry said in an earlier statement that Taiwan’s military tracked five of the ships as they sailed in coordination with the flights of the fighter jets.

China considers the self-ruled territory of Taiwan as its own and has vowed to bring it under its control in the future, using force if necessary. For the past three years, China has stepped up military pressure against Taiwan and has regularly held military missions around the island.