South Korea has condemned North Korea's latest verbal attack on its president as "unspeakable", after it likened her to a prostitute.
In remarks on Sunday, North Korea called President Park Geun-hye a "comfort woman", referring to sex slaves during World War Two.
It accused her of pandering to her "pimp", US President Barack Obama, who visited Seoul last week.
He and Ms Park warned Pyongyang against conducting a fourth nuclear test.
Their warning came after reports of increased activity at North Korea's nuclear test site.
In a statement, North Korea's Committee for the Peaceful Reunification of Korea accused Ms Park of catering to her "powerful pimp", Mr Obama.
It called Ms Park "a wicked sycophant and traitor, a dirty comfort woman for the US and despicable prostitute selling off the nation".
North Korean state media have carried personal attacks on South Korean leaders on many occasions in the past, but the rhetoric directed at Ms Park - the first female president - appears to be particularly vicious.
On Monday, Seoul's Unification Ministry spokesman Kim Eui-do urged Pyongyang to abide by an earlier agreement to stop the rhetoric.
He said the North's continuing "to issue unspeakable curses and foul words is an immoral act".
"If the North has the slightest feeling for its people... it should stop its senseless behaviour and take the path to co-operation and joint prosperity," he was quoted as saying.
Tensions have been high on the Korean peninsula amid reports based on satellite imagery that Pyongyang could be planning to conduct another nuclear test.
North Korea has carried out three such tests in the past, most recently in February 2013.
All resulted in the imposition of sanctions by the UN, which bars Pyongyang from nuclear tests under resolution 1718.
Last week, Mr Obama said that the US and South Korea stood "shoulder to shoulder, both in face of Pyongyang's provocations and our refusal to accept a nuclear North Korea".