North Korea is jamming GPS signals near the South Korean border, Yonhap news agency reports.
The radio waves have reportedly affected 110 planes and ships, and can cause mobile phones to malfunction.
South Korea’s unification ministry spokesman Jeong Joon-hee was quoted by AFP news agency as saying it was an “act of provocation”.
Tensions have been high between the two countries since North Korea’s fourth nuclear test in January.
The broadcasts appeared to have started a month ago from various locations along the border, but on March 31 North Korea discharged its largest amount of GPS-jamming signals, according to Yonhap, citing a senior government official.
The South Korean coastguard reported about 70 fishing vessels had been forced to return to port after GPS navigation issues, AFP said. There has been no reported disruption to flights.
According to Yonhap, since 2010 North Korea has been accused of jamming signals at least three other times. It is believed to be using equipment imported from Russia.
Pyongyang has called the allegations “sheer fabrication”.
Since North Korea’s nuclear test in January and subsequent launch of a long-range rocket, South Korea has stepped up security measures including border patrols and allowing the US to fly fighter jets near the border.
In return, North Korea has threatened “indiscriminate” nuclear strikes on the US and South Korea.