The United States Air Force spy plane came within 51 nautical miles of China's east coast as it passed through Beijing's air defense identification zone (ADIZ) on Thursday, flight tracking data showed.

The U-2A reconnaissance aircraft, nicknamed "Dragon Lady," took off from Seoul, South Korea, and made a sortie into the Taiwan Strait, according to the South China Sea Probing Initiative, a Beijing think tank.

The Cold War-era Lockheed spy plane traveled the length of the East China Sea ADIZ before turning back roughly 51 nautical miles from the eastern Chinese province of Fujian and about 70 nautical miles off Taiwan's northwest coast.

Data from the think tank suggests it was the closest reconnaissance flight in the Taiwan Strait by an official U.S. military aircraft since October, with Washington keen to ward off any attempts by China to act militarily against the self-ruled island of Taiwan.