They are based on MQM-107D Streaker target drones, which are used by the US army, and imported from a Middle East nation believed to be Syria, Yonhap news agency reported.
It cited an anonymous Seoul military official, adding the communist state would likely deploy them, once completed, near the tense maritime border with the South on the Yellow Sea.
The US drone, which flies at 40,000 feet (12,000 metres) at a maximum speed of 575 miles (920 kilometres) per hour, is commonly used for testing missiles.
The North has conducted several tests by mounting high explosives on the imported drones but has not been able to produce a new weapon yet, said the source quoted by Yonhap.
The disputed sea border off the west coast was the scene of deadly naval clashes in 1999, 2002 and 2009. The North also shelled a frontier island in a November 2010 attack that left four South Koreans dead.