North Korea is using radio waves to jam GPS navigation systems near the border regions, South Korean officials said.
The broadcasts have reportedly affected 110 planes and ships, and can cause mobile phones to malfunction.
The South'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 Koreas since the North's fourth nuclear test in January.
The broadcasts appeared to have started a month ago from various locations along the border, but on Thursday the North 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.
The South Korean news agency said that since 2010 the North 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".
Analysis: Stephen Evans, BBC News, Seoul
North Korea has jammed global positioning systems in South Korea before when tension between the two countries was high.
Four years ago, pilots on more than 300 commercial flights detected interference with their systems. Planes have back-ups, though, so no disaster happened.
This time, the South Korean government says 58 planes and 52 ships have been affected, again without serious damage so far.
But just the knowledge that GPS may be being interfered with from Pyongyang is still disorientating for ordinary people.
One South Korean told the BBC that the map on his phone suddenly put him far away from his actual position and he wondered if that was because of North Korea's actions.
Since the North's nuclear test in January and subsequent launch of a long-range rocket, the South has stepped up security measures including border patrols and allowing the US to fly fighter jets near the border.
The North in return has threatened "indiscriminate" nuclear strikes on the US and the South.