The South Korean military will begin live-firing exercises on an island close to the border with North Korea in the coming hours, its military says.
The move comes despite repeated threats of retaliation from Pyongyang.
Four people were killed when the North shelled the island during an earlier drill last month.
The UN Security Council has been discussing the situation in New York, but has failed to reach any agreement.
Speaking to reporters after more than eight hours of discussions, Russian ambassador Vitaly Churkin called on South Korea to cancel its plans.
“It’s better to refrain from doing this exercise at this point in time,” he said.
He said the UN had failed to reach an agreement on the crisis, but contacts would be pursued between the main powers.
The issue has threatened to divide permanent members of the Security Council – with China and Russia urging South Korea to put off the exercise, but the US saying its ally is entitled to make sure it is “properly prepared in the face of… ongoing provocations”.
The US ambassador to the UN Susan Rice said it was “safe to predict that the gaps that remain are unlikely to bridged.”
Residents on the island of Yeonpyeong have been told to move into air-raid shelters – part of regular procedure ahead of military exercises in the area.
Initially, it was reported that the exercise would begin at 0200 GMT, but it has now been put back to 0400 GMT, because of fog, according to a Defence Ministry spokesman.
The drill is expected to last for two hours.
Pyongyang has said it will retaliate if the South goes ahead with the exercise on Yeonpyeong – which is close to the two countries’ disputed sea border, the Northern Limit Line, and within view of the North Korean mainland.
It says it will deal an “unpredictable self-defensive blow” at the South Koreans, “deadlier” than when the North shelled the island during similar live-fire exercises on 23 November.
Two civilians and two marines were killed in that clash.
For its part, South Korea will “immediately and sternly” deal with any North Korean response, said an officer at the Joint Chiefs of Staff (JCS).
An unofficial US envoy – New Mexico Governor Bill Richardson – is in North Korea and has held several meetings with senior officials there. The situation is “very, very tense, a crisis situation”, he told CNN.
He was speaking after meeting North Korean Maj Gen Pak Rim-su, who leads North Korean forces along the border with the South.
That meeting was “very tough”, but “some progress” was made, Mr Richardson said.
“They said there would be a response, but at the same time they hope a UN Security Council resolution would tamp down the situation. It was very clear they were very upset by the potential exercise,” he told CNN from Pyongyang.
He suggested a military hotline be set up to address incidents along their border.
The BBC’s Jane O’Brien in Washington says the Obama administration is in a tough spot, as the US has 28,000 troops stationed in the South and it would almost certainly be drawn in if hostilities erupt
The US is walking a diplomatic tightrope, trying to avoid that unfavourable option while remaining a strong ally to the South, our correspondent adds.
The JCS has said that artillery guns on Yeonpyeong will be aimed south-west and away from North Korea for the drill.
But the North claims any ammunition fired would inevitably land in its territorial waters.
In the event of an attack from the North, the South’s Air Force would put its F-15K and KF-16 fighters on emergency standby, Yonhap quoted the JCS as saying.
The island is normally home to some 1,300 residents along with hundreds of marines, but most civilians have fled to the mainland, leaving only about 100 remaining, Yonhap said.
