Seoul warned North Korea Thursday to drop any plans to fire its longest-range missile, saying a launch would mean increased isolation and more sanctions for the communist state.

The North is apparently pushing ahead with preparations to launch its Taepodong-2, a South Korean news report has said, despite international appeals for restraint.

"If North Korea goes ahead with launching the missile, it will pose a serious threat to stability in Northeast Asia as well as inter-Korean relations," South Korea's foreign minister Yu Myung-Hwan told reporters.

"Such an act would isolate it... given various restrictions and sanctions which North Korea would incur with the missile launch, it would never be in the North's interest."

US and South Korean officials said last week the North seems to be preparing another test of its Taepodong-2, which could theoretically reach Alaska. The missile failed after 40 seconds when it was first test-fired in 2006.

The 2006 test, which also included shorter-range rockets, led to UN Security Council sanctions banning the transfer of missiles or missile-related items or technology.

North Korea in recent weeks has intensified threats against the conservative South Korean government of President Lee Myung-Bak, who has taken a tougher line on cross-border relations than his liberal predecessors.

Pyongyang has scrapped peace accords, nullified the sea border and warned of possible war.

The North has also staked out a tough stance in stalled nuclear disarmament negotiations involving the United States and four regional powers.

Analysts say the aim is to frighten Lee into dropping his harder line and to persuade President Barack Obama's administration to make the North one of its priorities.

Apparent plans for a missile test have the same purpose, they believe.

On Wednesday the North's leader Kim Jong-Il named a hawkish figure to the top military post in what observers said was a signal of a tougher policy towards the world.

Hillary Clinton, who Sunday starts her first Asia tour as US Secretary of State, warned Pyongyang this week against any provocation.

"We are making efforts to strengthen bilateral cooperation with the Obama administration to resolve the North Korean nuclear issue," foreign minister Yu said.