South Korean ambassador to the United Nations Oh Joon addresses the United Nations Security Council on North Korea at the United Nations Headquarters in New York March 2, 2016. REUTERS/Brendan McDermid

UNITED NATIONS (Reuters) - North Korea has been jamming GPS signals in South Korea since March 31, threatening the safety of civilian aircraft and vessels and violating international agreements, Seoul told the United Nations Security Council in a letter released on Monday.

South Korean U.N. Ambassador Oh Joon said the electronic jamming signals have come from five North Korean regions - Haeju, Yonan, Pyongyang, Kumgang and Kaesong - and “dangerously affect” the Global Positioning System.

“The GPS jamming by DPRK (North Korea) is an act of provocation that poses a threat to the security of the Republic of Korea and undermines the safety of civil transportation, including aircraft and vessels,” Oh wrote in the April 5 letter.

On April 1 South Korea warned North Korea to stop and vowed to take action if it continued amid heightened tension over the North’s nuclear and rocket tests.

The reclusive North and the rich, democratic South are technically still at war since their 1950-53 conflict ended in an armistice, not a peace treaty. Oh said the GPS jamming violates that armistice agreement.

South Korea has been on high alert against possible cyber attacks from the North after Pyongyang’s weapons tests and angry rhetoric threatening war in response to new sanctions imposed last month by the U.N. Security Council and the South.

“The government of the Republic of Korea strongly urges DPRK to stop its GPS jamming without further delay,” Oh wrote.