Two Chinese fighter jets flew within 50 meters of two Japanese surveillance aircraft over the East China Sea over the past weekend, according to defense officials with both countries.

Two Su-27 Chinese fighters intercepted a Japanese OP-3C surveillance plane and a YS-11EB electronic intelligence aircraft on Saturday near a string of islands in the East China Sea.

China and Japan have a dispute over the island chain because of the belief the area has vast oil resources undersea. The island chain is called the Senkaku Islands in Japanese and the Diaoyu Islands in Chinese.

In order to flex its military muscle and boost their hold on the island chain, China declared a no-fly zone over the region. The U.S. flew B-52 bombers without informing the Chinese when the no-fly zone area was declared, but later said U.S. pilots would inform China when their aircraft were in the region.

China’s intercept of the two surveillance aircraft is the first major aviation engagement since the no-fly zone was declared by China.

The Chinese state media reported that the two Su-27 fighter jets were flown out of China’s 7th Fighter Division.