SEOUL, South Korea - South Korea's president sought Wednesday to galvanize support from world leaders to pursue UN Security Council punishment for North Korea if it proceeds with a rocket launch that is suspected to be a cover for a missile test.

In one-on-one meetings in London on the eve of the G-20 summit, President Lee Myung-bak stressed the need for a "united response" among world leaders after Pyongyang carries out what it has said will be a satellite launch some time from Saturday to following Wednesday.

The U.S., South Korea and Japan believe the reclusive country is really testing its long-range missile technology, and they warn Pyongyang would face sanctions under a UN Security Council resolution that bans the country from any ballistic activity.

North Korea has refused to back down and issued warnings of its own, telling the U.S. it will shoot down any spy planes that intrude into its territory and threatening Japan that any effort to intervene in the launch would be considered an act of war.

"If the brigandish U.S. imperialists dare to infiltrate spy planes into our airspace to interfere with our peaceful satellite launch preparations, our revolutionary armed forces will mercilessly shoot them down," South Korea's Unification Ministry quoted North Korean radio as saying.

It is unclear what capability the North Korea has to shoot down high-flying Boeing RC-135s, which can reach altitudes of nearly 15 kilometres.

U.S. military officials in Seoul declined to comment on the spying allegations or the North's threat.

Lee and Japanese Prime Minister Taro Aso, meanwhile, reaffirmed their intention to take North Korea to the Security Council after the launch.

"A launch by North Korea would be a clear violation of a UN resolution," Aso told Lee on Wednesday, according to Osamu Sakashita, Aso's deputy Cabinet secretary for public relations. "On this issue, Japan, Korea and the United States need to work closely together," Aso said.

Lee assured Aso that South Korea supports Tokyo's right to take action to defend itself, Sakashita said. "He said Japan has every right to take measures to protect its citizens. Korea recognizes this," he said.

Japan has deployed battleships with antimissile systems off its northern coast and stationed Patriot missile interceptors around Tokyo to shoot down any wayward rocket debris that North Korea has said might litter the area.

The North has warned it would consider any interception "the start of Japan's war of re-invasion."

Japan says it is only protecting its territory and has no intention of trying to shoot down the rocket itself.

British Prime Minister Gordon Brown and Australian Prime Minister Kevin Rudd also sided with Lee, saying the North's launch would violate the UN resolution.

President Barack Obama is expected to join the push for a joint response when he meets with the South Korean leader Thursday. But it will be harder to convince President Hu Jintao of China, the North's only major ally, on Friday.

Beijing and Moscow -- veto-wielding permanent members of the Security Council -- could object to an attempt to seek UN sanctions against Pyongyang, citing legal uncertainty over the wording of the resolution because it makes "no mention of launches relating to peaceful outer space activities," the Brussels-based International Crisis Group said in a report.

"China and Russia might argue with at least equal plausibility" that the resolution relates only to military missile launches and programs, the group said.

U.S., South Korean and Japanese officials have stressed that missile and satellite launches use the same technology and differ only in payload, so a successful liftoff means the North has a way to launch nuclear warheads.

North Korea's "weapons of mass destruction combined with its ability to deliver something at a long range is a problem, regardless of what is mounted at the top of the rocket," Wi Sung-lac, Seoul's top nuclear envoy, said after returning from talks in Washington with his U.S. and Japanese counterparts.

Amid the tensions over the rocket launch, North Korea has said it will indict and try two American journalists -- Laura Ling and Euna Lee of former Vice President Al Gore's Current TV media venture -- for allegedly crossing the border illegally from China on March 17 and engaging in "hostile acts."

In Seoul, meanwhile, North and South Korea faced off again in another arena: the soccer pitch.

South Korea beat North Korea 1-0 in a World Cup qualifier Wednesday that drew frenzied cheers from hometown fans waving the South Korean flag.

"I hope we (South and North Korea) can step closer toward peace with this soccer game," said Kwon Jin-won, 21.