North Korea said yesterday its relations with South Korea had been driven into a "catastrophic" phase again, and warned the scattering of anti-Pyongyang leaflets could spark a war.
The statement came hours after Pyongyang's foreign ministry said the North would bolster its "war deterrent", and accused the United States of deliberately escalating tension through its ongoing military drills with the South.
Earlier, North Korea's military condemned the South Korean navy's seizure of a Northern fishing boat near the sea boundary as a "grave provocation" and threatened to retaliate.
South Korea handed back the boat, which was captured with three sailors on board near the disputed Yellow Sea border late on Thursday.
"North-South relations have been driven into a catastrophic phase again due to the South Korean authorities' frantic scattering of anti-DPRK [North Korea] leaflets", said a spokesman for the North's high-level delegation to the South.
"The leaflet scattering operation and smear campaign going beyond the tolerance limit are undisguised acts of declaring a war," he said.
The North's Committee for Peaceful Reunification of Korea said on Wednesday that the South's military scattered leaflets denouncing its regime and leader Kim Jong-un, by using balloons floated from islands near the border in the Yellow Sea.
The South Korean defence ministry denied the allegation. An unidentified military official told Yonhap news agency that the leaflets were launched by a local Christian group.
"Does she really want to see such leaflets becoming a source of war for reducing the base of provocations to ashes? She should bear in mind that now is the time to make a choice herself," the North's spokesman said, referring to South Korean President Park Geun-hye.
Pyongyang was also angered by a speech Park made on Monday at a nuclear summit in The Hague, in which she voiced concern that Pyongyang's nuclear material could end up in terrorist hands.
On Thursday, nearly 15,000 South Korean and US troops began a 12-day amphibious landing drill, the largest for two decades, a day after North Korea launched two medium-range ballistic missiles.
Code-named Ssang Yong (Twin Dragons), the exercise on the South's southeastern coast will last until April 7 and involve around 10,000 US troops.
North Korea views such exercises as provocative rehearsals for invasion, and there is a risk they could further fuel already simmering military tensions.
Pyongyang has carried out a series of rocket and short-range missile launches in recent weeks, sparking condemnation from Seoul and Washington.