South Korea expects North Korea to launch a rocket on Saturday if weather conditions permit, President Lee Myung-Bak said on Friday, while warning the move was "not in their benefit".

In an interview with a small group of journalists including AFP in London, Lee reiterated that Seoul would be ready to send an envoy to its northern neighbour if the reclusive state's leaders were willing to accept one.

North Korea's announcement that it will launch a communications satellite between April 4 and 8 has raised tensions in the region.

South Korea, the United States, Japan and others believe the reclusive communist regime will actually test a long-range ballistic missile which could eventually reach the US states of Alaska or Hawaii.

Asked when South Korea expected the rocket to be fired, Lee said it depended on weather conditions. "If tomorrow's weather is suitable for them, if they think it's suitable for them to test fire this missile then it's going to be tomorrow," he said.

"If the weather does not permit it, then I suspect it's going to be some other day."

He criticised North Korea's plan to fire a costly rocket at a time when it was suffering chronic food shortages which had left the country in a "dismal situation".

"I think it is important for the North Koreans to realise and fully understand that it is not in their benefit for them to test fire this missile," Lee said.

"Perhaps in the short-term it might give them an upper hand in the negotiations but in the long-term, their trust given to them by the international community will only lessen."

He stressed that if Pyongyang did fire the rocket, the regime of its leader Kim Jong-Il would violate a UN Security Council resolution passed after missile and nuclear tests in 2006 which bans it from any missile-related activity.

"If the North Koreans do test fire this... the members of the Security Council will take up this issue and accordingly they will be discussing the issue," he said, speaking through an interpreter.

"We favour sending out a very strong and stern message to the North Koreans that the international community does not condone, nor will it accept, North Korea engaging in such action".

Japan -- over whose territory the test is set to fly -- has said it may shoot down the rocket if it malfunctions, prompting the threat of retaliation with "deadly blows" from North Korea.

Tokyo is to call for an emergency meeting of the UN Security Council if Pyongyang goes ahead with the launch.

Lee also stressed South Korea's decision on whether to join the Profliferation Security Initiative (PSI) -- a US-led initiative to stop the spread of weapons of mass destruction -- would not be affected by North Korea.

"Whether my government takes part in the PSI or not, it is not going to be determined in any way by any threats emanating out of North Korea," he said. North Korea has threatened retaliation over any such move.

Speaking a day after world leaders gathered for a G20 economic summit in London, Lee said South Korea -- which chairs the group next year -- would exercise "perseverance and patience" to get North Korea to change.

"If North Korea is ready and willing to receive a special envoy from South Korea, of course we've always been ready to send one," he added.

During the summit, the former Hyundai executive and Seoul mayor met US President Barack for the first time and they vowed "stern, unified action" if the rocket is fired.

Lee said both he and Obama want change in status quo and that their both being in office offers a "watershed for transforming the Korean peninsula".

On the G20 summit, Lee said he was "very happy" to find that countries which had previously taken protectionist measures were "in the forefront" of calling for a renewed commitment against it, without naming names.

He said while some nations "may be tempted" to revert back to protectionism because of the current tough economic climate, they should "overcome such temptation".

"Reverting back to protectionist tendencies will not be to the benefit of any one country," said Lee, whose country is heavily export-dependent.