US President Barack Obama described North Korea as a pressing threat and pledged "unshakeable" support for South Korea after the communist state rained a deadly artillery barrage on a border island.

Washington and Seoul agreed to "coordinate" any response to North Korea after Tuesday's attack, which killed two South Korean marines and sent panicked civilians fleeing the flashpoint Yellow Sea island of Yeonpyeong.

In an interview with ABCNews, Obama would not speculate on military actions that the United States may take in response to the artillery deluge, which has incited global condemnation and depressed financial markets around the world.

But the "outraged" president was quoted as saying: "We want to make sure all the parties in the region recognize that this is a serious and ongoing threat that has to be dealt with."

South Korea, after decrying an "inhumane atrocity" against defenceless civilians, said Wednesday that it was suspending promised flood aid to North Korea, and has already called off talks on reuniting families split by war.

Obama was briefed by top security and military aides on the nuclear-armed Stalinist state's assault on the island, one of the worst border incidents since the 1950-1953 Korean war.

"The president reiterated the unshakeable support of the United States for our ally, the Republic of Korea, and discussed ways to advance peace and security on the Korean peninsula going forward," a White House statement said.

But both the United States, which has 28,500 troops in South Korea, and Seoul appeared bent on what officials called a "measured" response in tackling the latest spasm of tensions with North Korea.

"We're going to work with China, we're going to work with all our six-party partners on a response," State Department spokesman Mark Toner said, referring to an international group tackling North Korea's nuclear program.

The firing came after North Korea's disclosure of an apparently operational uranium enrichment plant -- a second potential way of building a nuclear bomb -- which is causing serious alarm for the United States and its allies.

It also comes as North Korea prepares for an eventual dynastic succession from Kim Jong-Il to his youngest son, Kim Jong-Un. The expected transfer is fueling speculation about the opaque regime's military and nuclear intentions.

Japan and Russia, both members of the six-nation group, have slammed North Korea's artillery raid.

Stephen Bosworth, a US nuclear envoy visiting Beijing on Wednesday, demanded that North Korea cease its "provocative" actions and urged "all members of the international community to condemn" Pyongyang.

China -- North Korea's sole major ally and economic prop -- has expressed "concern" over the shelling but has not publicly criticized North Korea.

The US-led United Nations Command, which monitors the uneasy 1953 armistice, called for general-level talks with North Korea to "de-escalate the situation."

South Korea's military went on top alert Tuesday, its troops fired back with cannon and the government convened in an underground war room after North Korea fired up to 50 artillery shells onto Yeonpyeong.

North Korea's supreme command, however, accused South Korea of firing first and vowed "merciless military attacks with no hesitation if the South Korean enemy dares to invade our sea territory by 0.001 mm".

"We ran for our lives. Both my shoes were pulled off my feet and I had to ride the boat barefoot," Han Mi-Soon, 52, said after fleeing to mainland South Korea aboard a ferry along with hundreds of other civilians.

Yeonpyeong lies just south of the border declared by UN forces after the war, 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 rejected the charge.

burs/jit/jm