A U.S. reconnaissance plane flew over the Korean Peninsula on Monday, three days after North Korea claimed to have tested an underwater nuclear weapons system.

According to multiple private aerial tracking websites and military authorities on Monday, the U.S. Air Force’s Boeing RC-135 flew over inland areas of Gyeonggi and Gangwon provinces as well as above the East Sea and West Sea earlier in the day for many hours. 

A military source said it is unprecedented for a U.S. recon plane to expose its tracks above the East Sea and West Sea for long hours. 

Some observers believe that the move comes after the military detected signs of additional provocations by the North. 

The plane’s dispatch came three days after Pyongyang said that it had conducted a test of the underwater weapon system called the “Haeil-5-23” in the East Sea in response to the recent maritime drills between South Korea, the U.S. and Japan.