The United States and South Korea stand "shoulder to shoulder" in their response to a deadly North Korean artillery bombardment and will soon stage combined war games, the White House said.
In their first joint response to Tuesday's attack on a South Korean border island, US President Barack Obama and South Korean President Lee Myung-Bak agreed on the military exercises, as pressure built on China to rein in its wayward ally.
South Korea, after decrying an "inhumane atrocity" against defenseless civilians, said it was suspending promised flood aid to North Korea, and has already called off talks on reuniting families split by the Korean War.
The attack on the Yellow Sea island of Yeonpyeong, which sent panicked civilians fleeing and depressed financial markets worldwide, has fueled anxiety about North Korea's intentions after a new nuclear program came to light.
Japan's Prime Minister Naoto Kan called on China to use its "significant influence over North Korea" to tamp down the latest spasm of tensions on the divided peninsula.
A White House statement said Obama telephoned Lee to declare that the United States "stands shoulder to shoulder" with its ally South Korea, which is home to 28,500 US troops.
The two leaders agreed to hold "combined military exercises and enhanced training in the days ahead," the statement said. South Korea's Yonhap news agency said that the two nations would start a naval exercise on Sunday.
The intention of the drills is to "continue the close security cooperation between our two countries, and to underscore the strength of our alliance and commitment to peace and security in the region," the White House said.
The artillery fire killed two South Korean marines and wounded 15 more plus three civilians in one of the worst incidents since the 1950-53 war, sparking outrage in many newspapers in Seoul as the government was urged to hit back.
"A club is the only medicine for a mad dog," the Dong-A Ilbo newspaper said, calling the shelling a "war crime" that demanded a military riposte.
South Korea readied to deploy more artillery on Yeonpyeong, including extra K-9 self-propelled guns to replace shorter-range 105-mm howitzers, after officials said North Korea fired up to 170 artillery shells into its territory.
"We're going to work with China, we're going to work with all our six-party partners on a response," US State Department spokesman Mark Toner said, referring to an international group tackling North Korea's nuclear drive.
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 ambitions.
China - North Korea's main ally and economic prop - has expressed "concern" over the shelling but not publicly criticized North Korea. Its media have given generally sympathetic coverage to Pyongyang's version of events.
North Korea's supreme command has 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".
But the rest of the world has united in blaming North Korea, and China is under mounting pressure to intervene, despite its historic reluctance to do anything to destabilise the Stalinist regime in Pyongyang.
"We should ask China, which has significant influence over North Korea, to make efforts to jointly restrain North Korean actions," Kan said at a Japanese cabinet task force meeting set up in response to the attack.
Australia called the "outrageously provocative" shelling a threat to the entire region's stability and Foreign Minister Kevin Rudd said, "I believe it's important now for China to bring all of its influence to bear on North Korea."
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 2009 November.
Tensions have been acute since the deadly 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.