North Korea on Saturday accused South Korea of violating its airspace with an unmanned vehicle, Pyongyang's state news agency reported.
According to KCNA, a Heron surveillance plane flew over the western border area four times between 7.46 a.m., and 8.40 a.m., on Friday.
The agency called the incident a "grave military provocation" as it coincided with the temporary deployment by the United States of four Global Hawk drones and around 100 soldiers at the Yokoda Air Base in Japan from their base in Guam.
Pyongyang also accused Seoul of escalating tensions and warned of a "merciless retaliatory response".
The incident took place after South Korea fired warning shots at the border on Tuesday at an unidentified object flying from North Korea to South Korea's Kangwon province, which it later identified as a propaganda balloon in support of the Kim Jong-un regime.
However, North Korean media said South Korean troops had fired at a flock of birds and accused them of being "reckless" and taking the confrontation between the two countries to a new level of "hysteria".
Repeated weapons tests by North Korea, including the most recent on May 20, have escalated tensions in the region and with Washington, who has hinted at possible pre-emptive strikes.
The weapons test took place barely a week after Pyongyang launched the Hwasong 12, a new medium range missile that indicates significant progress made by the Kim regime in developing an intercontinental missile with a nuclear warhead capable of reaching US territory.