North Korea urged an end to military hostilities Friday in the latest in a series of apparently conciliatory gestures to South Korea, which has so far reacted with scepticism.

The latest olive branch came in the form of an "open letter" sent to the South Korean authorities by the North's top military body on the direct orders of leader Kim Jong-Un.

South Korea said Thursday it was scrutinizing North Korea's military activities following a series of publicized army visits by leader Kim Jong-Un.

The North's state media has given prominent coverage in recent days to Kim's visits to two strategic units -- including a paratroop brigade -- both of which demonstrated their ability to destroy enemy installations "at lightning speed.”

North Korea remains Washington's "number one security concern" in Asia, the U.S. Pacific Fleet commander said Wednesday, despite simmering territorial disputes elsewhere in the region.

Admiral Harry B. Harris Jr also accused China of "coercion" in its maritime disputes with neighbors.

South Korean President Park Geun-Hye Saturday ordered the military to boost border security to protect the country against potential provocations by North Korea, unswayed by Pyongyang's latest peace overture.

Park's orders followed the North's surprise offer Thursday for an end to "all acts of provoking and slandering" from January 30 and its call for steps to ease tensions around the disputed Yellow Sea border, the scene of naval clashes in the past.

South Korea on Friday scoffed at North Korea's proposal for a mutual moratorium on verbal mud-slinging, and rejected Pyongyang's renewed calls to cancel planned military drills with the United States.

"We don't slander North Korea so there is nothing for us to stop," Unification Ministry spokesman Kim Eui-Do told reporters.

South Korea on Thursday rejected North Korean warnings to call off scheduled joint military exercises with the United States and vowed "severe" retaliation to any provocation from Pyongyang.

North Korea had urged Seoul to cancel the annual large-scale drills, known as Key Resolve and Foal Eagle, warning of "calamities and disaster" if they went ahead as planned starting late February.

North Korea warned Wednesday of "calamities and disaster" if the United States and South Korea push ahead with a series of annual joint military drills from next month.

The threat, which also said the drills would be tantamount to declaring nuclear war, came from the Committee for the Peaceful Reunification of Korea (CPRK), a North Korean state body in charge of inter-Korean affairs.

South Korean activists launched thousands of anti-Pyongyang leaflets and Wikipedia-loaded USB keys across the border Wednesday, despite past North Korean threats to shell the "human scum" involved.

Packages floated over the heavily-militarized border by balloon also contained 1,000 United States one dollar bills and DVDs detailing human rights abuses in the North.

South Korea declined to comment Wednesday on revelations that the United States talked it down from launching a retaliatory air strike on North Korea in 2010.

The claims were made in the newly published memoir of former U.S. defense secretary Robert Gates, in which he also describes former South Korean president Roh Moo-Hyun as "probably a little bit crazy.”

More than 1,500 North Koreans fled to South Korea last year, maintaining a recent fall in the number of escapees that coincided with a clampdown by new leader Kim Jong-Un.

Five years ago the annual number of escapees was close to 3,000, but the number dropped sharply after Kim came to power in December 2011, following the death of his father Kim Jong-Il.