WARSHIPS patrolled the Sea of Japan and Patriot batteries were set up around Tokyo yesterday as North Korea counted down to a missile launch intended to challenge President Barack Obama as he attends the G20 summit in London.Two Japanese guided-missile destroyers set sail under orders to intercept the Taepodong-2 if the launch goes wrong and it threatens to come down in Japan, a key US ally. North Korea has said any interception would amount to an act of war.

Kim Jong-il, the North Korean dictator, has hinted that if the missile is destroyed, his country will strike back violently, conduct a second nuclear weapons test and ruin years of American disarmament diplomacy.

North Korea, which conducted its first nuclear test in 2006, maintains that the Taepodong-2 is to launch a satellite into space for peaceful purposes. The US and Japan think it is a long-range missile designed for atomic warheads. Experts say the missile could be fired any time from today, although the North Koreans have set a date between April 4 and 8.

The launch has become a test of American power, according to one of the most senior foreign policy advisers in China. The US and Japan “will be bankrupt in reputation and dignity” if the missile violates Japanese sovereignty and is not destroyed, said Professor Zhang Lian-gui, of the Central Party school.

His comments, in an official journal, showed how keenly Chinese leaders were watching Obama’s performance under pressure. Obama will have his first summit with President Hu Jintao in London this week.

American and Chinese ships recently clashed in the South China Sea and the two nations exchanged angry words about a Pentagon report on China’s military build-up. The Chinese have refused to persuade their North Korean ally to call off the launch and are standing back to see how America and its allies deal with it.

The potential for error on both sides is high. The missile’s planned trajectory takes it soaring into space on an arc that leads across Japan. Previous North Korean missiles have exploded in flight or veered off course, and the US antimissile technology has not been perfected. Tokyo’s sophisticated Aegis vessels, the Kongo and the Chokai, which carry SM-3 interceptor missiles, were sent into the seas between the Koreas and Japan yesterday morning.

They will be joined tomorrow by two US Aegis destroyers, the USS John S McCain, skippered by a Korean-American naval officer, and the USS Chaffee. South Korea has sent its own Aegis destroyer, the King Sejong the Great.

On land, Japanese units deployed the latest Pac3 Patriot missile batteries to protect political and financial districts in Tokyo yesterday. The public was asked to stay calm. The Americans are also ready for the risk of a North Korean revenge strike across the border with South Korea.

On February 11, Kim demoted his chief of staff, General Kim Kyok-sik, to command the Fourth Army Corps in a move that may have been a strategic deception.

The corps guards the western coast, a tangle of islands and inlets where the border is disputed and where bloody naval battles between north and south claimed many lives in 1999 and 2002.

The state media announced that the outgoing chief of staff had also lost his seat in “elections” to North Korea’s rubber-stamp parliament. In retrospect, that seemed too simple. North Korea jealously guards its internal secrets and rarely discloses such a fall from favour. It may have been intended to deceive.

Analysts now suspect the “Dear Leader” may have deliberately placed the trusted man he wanted to command the west coast in preparation for another fight. The posting came less than two weeks after North Korea warned that it was scrapping all its accords with the south and would not respect the maritime borderline.

The former chief of staff will be perfectly versed in the north’s repertoire of fighting tactics. “Suicide operations, mini-submarines, teams of saboteurs, stealth, surprise and deception are all integral to their thinking,” said a US military source in Seoul.

As extra insurance, the North Koreans are holding captive two American video-journalists caught filming along the sensitive border with China. The two women, both of Asian origin, were working for a cable TV channel founded by the former vice-president Al Gore.

Diplomats said Kim was engaging in classic brinkmanship. Both China and Russia would almost certainly veto any United Nations security council resolution imposing new sanctions on North Korea, leaving the US and its allies with limited options. And Hillary Clinton, the US secretary of state, revealed in an interview with Fox News that Kim had refused even to let her new special envoy into the country.

For all the billions of dollars of multinational military hardware deployed around North Korea, its adversaries lack any hard facts about what Kim and his generals are planning. However, there have been reassuring words from a Russian expert on the North Korean leadership, Andrei Lankov. “They are not about to start a war,” he said. “Their major strategic goal is to die in their beds.”

Additional reporting: Sarah Baxter in Washington

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