South Korea's military fired warning shots after it spotted an unidentified object flying from North Korea across the Demilitarized Zone that divides the two countries, according to the South Korean Joint Chiefs of Staff.
The object, which the South Korean military said was no longer visible, was spotted two days after Pyongyang test-fired a ballistic missile -- its second missile test in seven days.
It's not the first time South Korea has reacted to an object sent across its border.
The South responded with machine-gun fire after an unidentified North Korean aircraft crossed the DMZ early last year. It "immediately went towards the North," a South Korean official said at the time.
North Korea is believed to have a fleet of more than 1,300 fighter jets, as well as an unspecified number of its own unmanned aerial vehicles, according to a 2015 report to Congress from the US Department of Defense.
Two suspected North Korean drones were found in South Korea in 2014, one on the island of Baengnyeong and another in the city of Paju, close to the border.
'Flagrant and provocative defiance'
The United Nations Security Council is set to meet Tuesday morning to discuss North Korea's recent actions, including the Sunday missile test.
Its members condemned the launch in a statement, calling it highly destabilizing behavior in "flagrant and provocative defiance of the Security Council."
The situation on the Korean peninsula has intensified in recent months, with the nascent administration of US President Donald Trump vowing to pressure Pyongyang to the negotiating table. Kim Jong Un's regime has responded defiantly, forging ahead with its missile tests, though the country has held back on conducting another nuclear test.