Dozens of fishing ships have returned to port in South Korea after the North disrupted their satellite navigation systems, officials in Seoul said. Pyongyang also announced it was banning Facebook and Twitter.
A North Korean jamming signal has been disrupting GPS reception on ships and planes to its south, the South Korean authorities said on Friday.
At least two planes reported the jamming, but no flights were affected. Also, over 70 fishing vessels returned to port due to navigation issues, according to coastguard officials.
"GPS jamming is an act of provocation," Unification Ministry spokesman Jeong Joon-Hee told journalists."We urge the North to stop such provocative acts and behave in a manner that would help improve inter-Korean relations."
According to the science and communication ministry, the signal was first detected on Thursday and the North allegedly boosted it on Friday. However, it was still too weak to cause a wider disruption, the Yonhap News Agency reported.
"If the North's GPS jamming results in real damage to airplanes or ships, we will make the North pay a due price," ministry spokesman Moon Sang Gyun was quoted as saying.
Facebook banned in Pyongyang
Also on Friday, the Pyongyang government announced a block on Facebook, Twitter, Youtube, Voice of America and a range of South Korean websites.
Although very few North Koreans have Internet access, foreigners visiting the isolated country had been allowed to go online with few restrictions. During recent months, however, Facebook and Twitter have been informally blocked and unavailable from Pyongyang.
Instagram was not on the list of banned social media and was functioning normally on Friday.
A number of North Korean sites are also restricted in the south of the peninsula.
New missile fired
Tensions between the North and the South have been especially high for months, following Pyongyang's nuclear test in January. The North has also been launching short and medium range missiles and boasting an alleged hydrogen bomb capability. Recently, its propaganda department published a video depicting a nuclear strike on Washington.
South Korean military detected another short-range missile launch off North Korean coast on Friday.
US nuclear talks 'nonsensical'
The latest escalation from Pyongyang comes as the US hosts a summit on nuclear security, with leaders from South Korea, Japan and China in attendance.
Before meeting with Chinese president Xi Jinping on Thursday, US President Barack Obama announced that the two politicians would talk about "how we can discourage actions like nuclear missile tests that escalate tensions and violate international obligations."
The United States has long urged China, an ally of North Korea, to press North Korea to give up its nuclear weapons program.
Obama also talked to leaders of Japan and South Korea in a trilateral meeting.
Pyongyang's state media has labeled the summit a "nonsensical" effort to find fault with the North's "legitimate access to nuclear weapons."