South Korea has warned North Korea it would "sternly retaliate" for any further provocations after dozens of shells were fired at a South Korean island on Tuesday, the presidential office said.
"Our military... will sternly retaliate against any further provocations," a presidential statement said.
"North Korea's shelling of Yeonpyeong island constitutes a clear armed provocation. Furthermore, its reckless shelling of civilian targets is unpardonable.
"North Korean authorities must take responsibility."
North Korean state media claims South Korea had fired first, prompting it to take military action.
"Despite our repeated warnings, South Korea fired dozens of shells from 1pm ... and we've taken strong military action immediately," the North's official KCNA news agency said in a brief statement.
It did not elaborate whether North Korea had suffered any damage from the exchange of fire.
The UAE's foreign ministry has condemned the shelling by North Korea. According to the state news agency WAM, in a statement issued by the ministry today, the foreign minister Sheikh Abdullah bin Zayed described the attack as an "irresponsible act" and called on the concerned parties to exercise self-restraint and return to a dialogue.
WAM also said Sheikh Abdullah had affirmed the UAE's support for the government and people of South Korea.
North Korea fired dozens of artillery shells onto the island, killing two South Korean marines and triggering an exchange of fire as southern armed forces went on their highest state of alert. The White House said it "strongly condemns" the attack and was "firmly committed to the defence of our ally, the Republic of Korea, and to the maintenance of regional peace and stability".
Announcing the deaths of the two marines, General Lee Hong-Ki also told a briefing that five marines were seriously hurt in the artillery barrage and 10 suffered minor injuries, while three civilians were hurt.
"It was a pre-planned, intentional and illegal attack in violation of the United Nations ceasefire accord, and an inhumane atrocity that fired random shells towards residential areas of defenceless civilians," a stern-faced Lee said.
He said the South's intensive return fire must have inflicted "significant damage" on the North's batteries. Officials said the South fired 80 rounds from its K-9 self-propelled guns on Yeonpyeong island.
Fires were burning out of control and spreading, a witness said. YTN television quoted a witness as saying that many of the North Korean shells had landed at military base on the island.
In what appeared to be one of the most serious border incidents since the 1950-53 war, South Korea’s government convened in an underground war room and air force jets were reportedly scrambled to the Yellow Sea island. Japanese Prime Minister Naoto Kan said today he has ordered his ministers to prepare for any eventuality.
"I ordered (ministers) to make preparations so that we can react firmly, should any unexpected event occur," Mr Kan told reporters after an emergency meeting of cabinet members and senior officials at his official residence.
"I ordered them to do their utmost to gather information," he said.
The firing came after North Korea’s disclosure of an apparently operational uranium enrichment programme - a second potential way of building a nuclear bomb - which is causing serious alarm for the United States and its allies.
Some 50 North Korean shells landed on the South Korean border island of Yeonpyeong near the tense Yellow Sea border, damaging dozens of houses and sending plumes of thick smoke into the air, YTN television reported.
One South Korean Marine - part of a contingent based permanently on Yeonpyeong island - was killed, the military said.
The military said 13 Marines were injured and YTN said two civilians were also hurt.
“A North Korean artillery unit staged an illegal firing provocation at 2:34 pm (0534 GMT) and South Korean troops fired back immediately in self-defence,” a ministry spokesman said.
“A Class-A military alert issued for battle situations has been imposed immediately,” the spokesman said.
One island resident, Lee Jong-Sik, told YTN: “At least 10 houses are burning. I can’t see clearly for the smoke. The hillsides are also on fire.
“We were told by loudspeakers to flee our homes.”
Residents of the island described scenes of terror. "Some time after our own military staged an artillery exercise, shells from the North started falling into our island," said Woo Soo-Woo, 62, a guest house owner on Yeonpyeong island near the flashpoint Yellow Sea border. "Flashes along with a thundering sound were seen here and there across our villages and up to 10 houses were engulfed in flames. Black smoke billowed around houses," Mr Woo said by mobile phone from a ferry heading to Incheon port.
The shooting started bushfires at several places in the hills, he said. "Frightened villagers rushed to nearby shelters while others were busy running away and crowded the port to escape."
Mr Woo said some 1,500-1,700 civilians live on the island, along with a permanent marine detachment. He had no information on civilian casualties. He said it was the first time he could remember North Korean shells falling on the island.
"I was at home but suddenly heard a thunderous sound outside. When I walked out, the whole village was on fire," another villager was quoted by Yonhap news agency as saying. "I'm at the evacuation site with other villagers and I am scared to death."
Another island resident, Lee Jong-Sik, told YTN television: "At least 10 houses are burning. I can't see clearly for the smoke. The hillsides are also on fire.
"We were told by loudspeakers to flee our homes for bunkers."
Yeonpyeong lies just south of the border declared by United Nations forces after the inconclusive war six decades ago, but north of the sea border declared by Pyongyang.
The Yellow Sea border was the scene of deadly naval clashes in 1999, 2002 and last November.
Tensions have been acute since the sinking of a South Korean warship in March, which Seoul says was the result of a North Korean torpedo attack. Pyongyang has angrily rejected the charge.
In late October, North and South Korean troops exchanged fire across their Cold War border, coinciding with a state of high alert for the South’s military in the buildup to the G20 summit of world leaders in Seoul earlier this month.
South Korean President Lee Myung-Bak convened an emergency security meeting in response to the latest incident, a presidential spokesman said.
“He is now in an underground war room to discuss possible responses with ministers of related agencies and national security advisers,” the spokesman said.
Lee urged the officials to “handle it (the situation) well to prevent further escalation”, the spokesman said.
The firing comes after Kim Jong-Un, the little-known youngest son of Kim Jong-Il, was officially recognised as number two in North Korea’s political system, clouding outsiders’ view of its military and nuclear intentions.
The new crisis erupted as a US special envoy headed to China Tuesday to seek its help in curbing North Korea’s new nuclear project, revealed to US experts who described a sophisticated programme to enrich uranium.
Stephen Bosworth has also visited South Korea and Japan this week to discuss the disclosure, which US officials say would allow the isolated North to build new atomic bombs.
Bosworth, speaking in Tokyo, ruled out a resumption of stalled six-nation talks - aimed at disarming the North of nuclear weaponry in return for aid and other concessions - while work continues on the enrichment programme.
China chairs the talks and is also the North’s sole major ally and economic prop. It has come under pressure to play a leading role in resolving the latest nuclear dispute.
China appealed for the six-party talks to resume after the new revelations, and expressed concern over Tuesday’s cross-border firing.
“We have taken note of the relevant report and we express concern over the situation,” foreign ministry spokesman Hong Lei told reporters.
“We hope the relevant parties do more to contribute to peace and stability on the Korean peninsula,” he said. Russia also warned against an escalation of tensions on the peninsula.
Russia warned against an escalation of tensions after North Korea fired artillery shells onto a South Korean border island, a foreign ministry official told Interfax.
"It's important that this does not lead to an aggravation of the situation on the Korean peninsula," the official, who was not named, told Interfax.