North Korea's nuclear weapons are "not a plaything" and their future is not up for negotiation, Pyongyang's ambassador to China said Tuesday, ahead of a visit by a U.S. envoy.

The comments by Ji Jae-Ryong came after an international deal with Tehran to curb Iran's nuclear capabilities in return for the lifting of crippling sanctions.

South Korean police on Monday blocked activists from launching anti-Pyongyang leaflets across the heavily fortified border, after fresh North Korean warnings about dire consequences if such events went ahead.

Dozens of activists had planned to launch helium-filled balloons carrying about 30,000 leaflets criticizing the North's ruling Kim family.

North Korea said Tuesday it had no interest whatsoever in following Iran down the path of nuclear dialogue, insisting it was already a nuclear weapons state and therefore beyond any logical comparison with Tehran.

One week after the historic deal that cleared a path to lift sanctions crippling Iran's economy in exchange for limits on its nuclear program, Pyongyang rejected any suggestion that it might follow suit.

South Korea voiced regret Monday at North Korea's rejection of its offers for talks at various levels as both countries prepare to mark the 70th anniversary of the Korean peninsula's liberation from Japanese colonial rule.

Last week, Seoul's parliamentary speaker Chung Ui-Hwa had used the August 15 anniversary as a peg to offer talks with his North Korean counterpart, Kim Yong-Nam.

State-controlled local elections in North Korea on Sunday saw a 99.97 percent voter participation rate, state media reported, in polls to elect new representatives put forward by the ruling party.

Typically, 99 percent of North Korean voters in the de facto single-party state take part in elections and 99 percent of them cast "yes" votes for uncontested candidates.

South Korean troops fired warning shots at North Korean soldiers who intruded on the buffer zone marking their heavily armed border, Seoul said Sunday, in the first such skirmish this year.

The incident occurred Saturday morning when 10 North Korean soldiers crossed the border of the demilitarised zone separating the two Koreas, which remain technically at war, the South's defence ministry said.

Domestic flights were cancelled in South Korea Sunday as Typhoon Chan-hom brought strong winds and heavy rainfall to southwestern provinces, after wreaking substantial damage in China.

The storm grounded more than 150 domestic flights nationwide, airport authorities said. International flights were not affected. 

North Korea responded angrily Thursday to reports in the South Korean media about a recent spate of high-level defections and executions, calling them Nazi-style propaganda aimed at tarnishing Pyongyang's image.

The reports in a number of publications have been accompanied by speculation that the defections signal a growing instability in the North Korean leadership under Kim Jong-Un.

North Korea on Wednesday threatened to retaliate should the South fail to return three of its citizens rescued from a drifting fishing boat.

A South Korean coastguard vessel picked up the five fishermen last Saturday after their boat was found drifting across the sea border off the eastern coast.

Three North Koreans wanted to defect after they were rescued from a small fishing boat which had drifted into South Korean waters off the east coast, officials said Tuesday.

A South Korean coastguard vessel rescued four fishermen and a farmer from North Korea last Saturday after their boat was found drifting across the sea border, the South's unification ministry said.