South Korea accused North Korea Friday of "crossing the line" with its recent threats and insults, and pressed the impoverished country to start repayments for past food aid.
The unification ministry, in charge of cross-border affairs, said the North should immediately halt threats and slander against the South's conservative President Lee Myung-Bak.
"Such threats are crossing the line the two Koreas must keep in their relations," said spokesman Kim Hyung-Suk.
The North has for months heaped insults on Lee and other conservative leaders, terming them "rats" and "human scum".
It threatened "sacred war" against the South in retaliation for perceived insults during Pyongyang's April commemoration of the centenary of the birth of founding leader Kim Il-Sung.
On Monday Pyongyang's military threatened rocket attacks on the Seoul offices of seven media outlets in revenge for their critical coverage of a mass children's event north of the border this week.
It listed the coordinates of some of the offices and said missile units and other forces had already targeted the buildings.
"We once again demand gravely that North Korea immediately stop such provocative acts," the ministry spokesman said.
He also urged Pyongyang to start paying for food aid shipped by Seoul between 2000 and 2007, saying the first payment of principal and interest worth $5.83 million was due on June 7.
"The position of our government is that loans for food aid... must be repaid as agreed, and we demand North Korea keep the agreement sincerely," the spokesman said.
He said Seoul could take further steps if Pyongyang fails to pay up but did not elaborate.
Seoul sent a notice through its Export-Import Bank in May giving advance notification of the date for the first payment. It sent a second notice on Friday.
Under the previous left-leaning government, the South provided the North with some 2.6 million tonnes of food worth $720 million in six instalments between 2000 and 2007.
Repayments for the food were to be made in the form of a long-term cheap loan, while fertiliser worth a total of 799.5 billion won ($683 million) was given free from 2000 to 2007.
The South also lent the North equipment and material worth $140 million for railways and roads, and another $88 million for developing light industry and natural resources.
The food and fertiliser aid ended after President Lee took office in early 2008 and rolled back the "sunshine" policy of aid and engagement with the North.