South Korea put its air force on alert as it prepared to start a live-fire exercise on a border island Monday, despite North Korean threats of deadly retaliation.

An emergency UN Security Council meeting failed to agree a statement on the crisis, and Russia warned that the international community was now left without "a game plan" to counter escalating tensions.

A defence ministry official told AFP the air force had been readied in case of any North Korean provocation.

He did not confirm a Yonhap news agency report that two destroyers have also been deployed in forward positions in the Yellow Sea.

The military ordered civilians on five frontline islands to take shelter, but a defence ministry spokesman said sea fog had delayed the start of the artillery drill on Yeonpyeong island.

After a similar exercise by marines based on Yeonpyeong on November 23, the North fired some 170 shells onto or around the island, killing four people including civilians and damaging dozens of homes.

It has threatened even deadlier retaliation if the upcoming drill goes ahead, saying South Korean shells from such exercises regularly land in its waters.

The North disputes the Yellow Sea border drawn by United Nations forces after the 1950-53 Korean War. It claims the waters around Yeonpyeong as its own.

The North's military appears to be preparing for a counter-attack, removing covers from coastal artillery guns and forward-deploying some batteries, a military source told Yonhap.

Apart from the military there are about 280 civilians including reporters on Yeonpyeong, which is 12 kilometres (seven miles) from the North's coast.

At the UN, China fended off Western demands that its ally North Korea be publicly condemned for the November 23 artillery assault, diplomats said.

They said it even rejected a proposed statement which did not mention North Korea or the name of Yeonpyeong.

"Now we have a situation with very serious political tension and no game plan on the diplomatic side," said Russia's UN envoy Vitaly Churkin, after the failure of eight hours of formal negotiations, and private talks which included the ambassadors from both Koreas.

The foreign ministers of China and Russia held telephone talks Saturday and urged South Korea to cancel its military exercise. But its ally the United States defended its right to self-defence.

Last month's bombardment was the first of civilian areas in the South since the war. It sparked outrage in the South, which rushed more troops and guns to the frontline islands.

About 20 US soldiers -- part of a 28,500-strong force stationed in the South -- are on Yeonpyeong to provide back-up in the latest drill, the US military said.

Five delegates from the United Nations Command and the Military Armistice Commission, which supervises the truce that ended the war, are also on the island, another spokesperson said.

They include personnel from Britain, Australia and New Zealand.

The North said Saturday the upcoming exercise "would make it impossible to prevent the situation on the Korean peninsula from exploding and escape its ensuing disaster".

Pyongyang, deriding the US troops as a "human shield", said its military has already threatened "decisive and merciless punishment" and "does not make an empty talk".

South Korea, heavily criticised for a perceived weak response to last month's bombardment, has vowed to hit back hard against any new attack.

It says the exercise is a routine defensive drill, with guns pointed away from the North and shells landing 10 kilometres (six miles) south of the sea border known as the Northern Limit Line.

"If we do not go ahead with the exercise, it means we will never be able to stage a firing drill near our five northern islands in the Yellow Sea," a senior presidential aide told JoongAng Daily.

"That means the Northern Limit Line will lose its meaning, and that means the capital region (Seoul) will be under threat. That's precisely what North Korea wants."

President Lee Myung-Bak called for national unity. "If opinion is split, the opponent will try to take advantage of it, no matter how strong and superior our defence power is," he said.

The Yellow Sea border was the scene of deadly naval clashes in 1999, 2002 and November 2009.

In March this year a South Korean corvette sank near the border with the loss of 46 lives. The South said a North Korean torpedo downed the ship, a charge denied by Pyongyang.