South Korean troops fired dozens of rounds on Tuesday at what appears to have been a drone that crossed the Demilitarised Zone from North Korea and entered the South's air space.

South Korea's Joint Chiefs of Staff said the aircraft was detected flying south in the eastern province of Gangwon at around 4pm, Yonhap News reported.

Around 90 machine gun rounds were fired as warning shots, while a warning was also issued over a loudspeaker system that broadcasts into the North.

The drone is believed to have returned to the northern side of the DMZ, although the South Korean military has stepped up its defence posture along the border.

The incident comes just five days after Moon Jae-in, the newly inaugurated South Korean president, warned of a "high possibility" of clashes along the border as tensions run high over Pyongyang's development of missiles and nuclear weapons.