The UN Security Council is holding an emergency session to discuss escalating tensions on the Korean peninsula.
Russia, which requested the talks, says it wants the UN to send a "restraining signal" to both North and South Korea.
The South insists that, weather permitting, it will go ahead with a controversial military exercise this week near a disputed maritime border.
The North condemned the drill - to be held on an island it shelled last month - and has threatened to retaliate.
The issue threatens 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".
'Crisis'
The South Korean military's determination to hold the drills remains unchanged, an unnamed defence ministry official said.
"There is no plan to cancel the exercise. The factor we're looking at is the weather condition," the official said.
Pyongyang is threatening to retaliate if the South goes ahead with planned military exercises on Yeonpyeong island - 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.
Four people - two civilians and two marines - were killed in that clash.
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.
Tightrope
The BBC's Jane O'Brien in Washington says the Obama administration 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 South's Joint Chiefs of Staff (JCS) said Saturday that artillery guns on Yeonpyeong will be aimed south-west and away from North Korea for the drill, reported South Korean news agency Yonhap.
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, adding the exact timing of the drill will be announced hours before it begins, depending on weather conditions.
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.
