A full-scale US and South Korean military exercise designed to deter North Korea ended without incident Wednesday despite Pyongyang's threats of possible nuclear retaliation.

The two allies, who accuse the North of a deadly submarine attack on a South Korean warship, said they staged the four-day naval and air exercise to send a "clear message" that any future provocations would not be tolerated.

About 20 ships including the 97,000-ton carrier USS George Washington, 200 aircraft including four F-22 Raptor stealth fighters, and 8,000 personnel took part in the largest joint drill for years.

"We practised well together and the (South's) military has built confidence that it can deter and defeat any North Korean aggression at any time, based on its alliance with the US," an official with Seoul's Joint Chiefs of Staff said.

The exercise which ended at 5:00 pm (0800 GMT) was a "formidable show of force" to North Korea, he told reporters in a background briefing.

The North vehemently denies involvement in the sinking of the warship in March with the loss of 46 lives. It had threatened nuclear retaliation against the drill, which it depicted as a rehearsal for invasion.

Seoul's military said no unusual military moves have been detected across the border since Sunday.

But the presidential office said the National Cyber Security Center had received intelligence reports of a possible cyber attack from the North, following its vow to hit back for the exercise.

A presidential team "has been on emergency alert against hacking in cooperation" with the centre, a presidential spokeswoman said.

South Korea's spy chief blamed North Korea for cyber attacks from China-based servers that briefly crippled US and South Korean government and commercial websites in July last year.

Tensions remain high on the peninsula almost six decades after an armistice ended the 1950-53 Korean War. It was never followed by a peace treaty.

North Korean leader Kim Jong-Il attended a concert marking Tuesday's 57th anniversary of the armistice, Pyongyang's official news agency said early Wednesday.

Kim hailed his country's "shining victory" in the conflict and enjoyed numbers such as "Our General Is the Best" and "July 27, Our Victory Day", it said.

The exercise which ended Wednesday in the Sea of Japan (East Sea) focused on defences against submarines. Seoul's defence ministry has said some nine other joint drills will be held later this year.

The United States has also announced new sanctions to punish the North for the sinking and push it to scrap its nuclear weapons programme.

Robert Einhorn, the State Department's special adviser for non-proliferation and arms control, will arrive in Seoul this weekend, Yonhap news agency quoted a diplomatic source as saying.

South Korean officials will meet the Americans Monday to discuss financial sanctions and other penalties against the North, the source said. The dates could not immediately be confirmed.

"We're looking to identify front companies which help North Korea evade existing sanctions," State Department spokesman Philip Crowley said Monday.

related report
Japan to delay defence paper to avoid S. Korea row
Japan will delay the release of an annual defence white paper to avoid angering South Korea over a territorial row ahead of the centennial of Japan's annexation of Korea, reports said Wednesday. The rare decision reflects Tokyo's eagerness to avoid any action that could provoke South Korean protests ahead of the August 29 anniversary of Japan's takeover of the Korean Peninsula, Kyodo News said. The government confirmed the delay, saying that officials needed more time to write about the deadly March sinking of a South Korean warship, which a multinational investigation team blamed on North Korea. The annual white paper, which was set to be released Friday, routinely includes a passage saying Japan and South Korea have an unresolved territorial dispute over small islands called Takeshima in Japan and Dokdo in South Korea. Japan's position has been that the South Korea-controlled islands in the Sea of Japan, also known as the East Sea, are an "inherent part of the territory of Japan," angering Seoul, which also claims the islands. Tokyo officials have reportedly expressed concern that anti-Japan sentiment in South Korea may intensify this year as Seoul commemorates the anniversary of Japan's annexation of Korea in 1910, which led to a brutal occupation that ended with Japan's defeat in World War II. Chief Cabinet Secretary Yoshito Sengoku has decided to delay the release of the annual white paper, according to major media including the liberal Asahi Shimbun and the conservative Sankei Shimbun. At his regular press briefing, Sengoku rejected that explanation and said the delay was due to the sunken South Korean warship. The government "decided to publish the defence white paper in September, considering the recent situation," he said. The South Korean government had asked Japan to drop a description of the disputed islands as Japan's from this year's white paper, Kyodo said, citing unnamed sources, but said Japan would not delete its claim to the islands. The decision to delay the report could spark criticism from Japanese conservatives who would see such a move as weak-kneed diplomacy, Kyodo said. The government of Prime Minister Naoto Kan was mulling issuing a special statement about the anniversary, the Asahi said.