Seoul:  North Korea has sent jet fighters to test the South's air defences and threatened to reduce its wealthy neighbour to ashes as it tries to push the new government in Seoul to back off from its hard line with Pyongyang.

Yesterday, the South's biggest daily newspaper said the communist state's jets had flown sorties close to Seoul's airspace at least 10 times since Lee Myung-bak became president on February 25. The North said at the weekend it was ready to attack.

"These should be understood as the first actions signalling a freeze in North and South Korean relations," said Yang Moo-jin, a specialist on North Korea at the South's Kyungnam University.

Lee's government has warned Pyongyang that if it wants to keep receiving aid it should improve human rights, abide by an international nuclear deal and start returning the more than 1,000 Southerners kidnapped or held since the 1950-53 Korean War.

Lee's stand has infuriated the touchy North, used to taking in billions of dollars over the past 10 years from Lee's left-of-centre predecessors who asked for little in return under their so-called "sunshine policy" of engagement, analysts said.

Forcing China's hand

But analysts added that China, one of North Korea's biggest benefactors and the closest the destitute state can claim to a major ally, would lean on the hermit state to prevent the situation on the Korean peninsula spinning out of control.

Beijing, already facing criticism for its handling of the crises in Tibet and Sudan, does not want North Korea to be another headache and spoil its hosting of the 2008 Summer Olympics, they said.

"China doesn't want any turmoil," said Kim Sung-han, a Korea University professor and expert in international relations.

China may be willing to step in and make up the difference in the hefty amount of food and monetary aid Pyongyang loses due to a blip in ties with Seoul, its other main benefactor, in order to keep things quiet for the Games, he said. Last week, North Korea expelled South Korean officials from a joint factory park just north of the heavily armed border once hailed as a model of economic cooperation.

It then test-fired missiles into the Yellow Sea, following that up at the weekend with a threat to launch a pre-emptive strike to "not merely plunge everything into flames, but reduce it (the South) to ashes".

It has also threatened to attack South Korean naval vessels patrolling in disputed Yellow Sea waters, suspend inter-Korean dialogue and stop taking apart its nuclear weapons plant as called for under an international deal.

A pro-Pyongyang newspaper published in Japan said relations between the two Koreas have fallen to lows last seen when North Korea tested a nuclear device in October 2006.

Analysts say the North could next cut inter-Korean projects, slow down or shut the joint factory park in Kaesong and come up with a new set of provocative military moves.

The North stations most of its ill-equipped 1.2-million-man army near the border, which is just 50 km from Seoul. The head of US forces in South Korea says the 28,000 US troops in the South who support the country's 670,000-strong military could easily defeat an invading North Korean army.

The North Korean moves come as conservatives are trying to win control of the South's parliament from left-of-centre forces in an April 9 election. If conservatives win a majority, it would significantly strengthen Lee's hand during his five-year term.