South Korea's military began a major live-fire exercise on Monday amid high tensions sparked by North Korea's deadly bombardment last month, as Washington pressed Beijing to curb its unruly ally Pyongyang.
The South, angry over the unprecedented peacetime shelling, dismissed claims by the North that the five-day drill could spark war.
The land, sea and air firing exercise follows last week's major show of naval strength by Seoul and its close ally Washington, designed to deter Pyongyang from future attacks.
US President Barack Obama, in a telephone call late Sunday, urged his Chinese counterpart Hu Jintao to send "a clear message" to the North that its shelling of South Korean territory and other provocations must end.
Mr Obama urged China to work with the United States and other countries "to send a clear message to North Korea that its provocations are unacceptable", a White House statement said.
The North's bombardment, the first of civilian areas in the South since the 1950-1953 war, came less than two weeks after it put a new and apparently operational uranium enrichment plant on show to US visitors.
The North says the plant is for peaceful purposes. But US experts and officials say it could easily be reconfigured to make weapons-grade uranium to supplement an existing plutonium stockpile.
US secretary of state Hillary Clinton will meet foreign ministers Kim Sung-Hwan of South Korea and Japan's Seiji Maehara later Monday in Washington to forge a strategy for dealing with the North.
"The three foreign ministers will discuss the North Korean nuclear issue, the issue of deterring North Korea from provocations and ways to lead North Korea to become a more responsible member of the international community," said Seoul's foreign ministry spokesman.
All three nations have responded coolly to a proposal by China, the North's sole major ally, for an emergency meeting of envoys to stalled six-nation nuclear disarmament talks to discuss the Korean peninsula crisis.
China's refusal to blame the North for the November 23 bombardment of a South Korean border island — which killed two civilians and two marines and caused widespread damage — has caused frustration among the United States and its allies.
The Washington Post reported that the Obama administration has privately started accusing Beijing of "enabling" North Korea to start the uranium enrichment programme and to launch attacks on South Korea.
Hu in talks with Obama stressed the need for a "cool and rational" response from all sides, Beijing's foreign ministry said. "Under the current situation, it is very possible that tensions on the Korean peninsula could further escalate and get out of control if not properly handled," it quoted him as saying.
Seoul's Joint Chiefs of Staff said the five-day exercise which began Monday would take place in 29 locations off the South. But a spokesman said bad weather had delayed one of the drills, off Daecheong island near the flashpoint disputed Yellow Sea border. He said it could go ahead later in the week.
Pyongyang said it shelled Yeonpyeong island in retaliation for a South Korean artillery drill, which allegedly lobbed shells into waters claimed by the North.
"The frantic provocations on the part of the puppet group (Seoul government) are rapidly driving the situation on the Korean peninsula to an uncontrollable extreme phase," its official news agency said Sunday.
"I don't dwell on North Korea's response as it does not deserve even a little consideration," said the South's new defence minister Kim Kwan-Jin, who has vowed to hit back with air strikes against any new attack.
More troops and weaponry are being sent to five frontline islands "so that we will be able to respond resolutely to any provocations", Prime Minister Kim Hwang-Sik said.
He said the government would spend 30 billion won ($26 million) to help Yeonpyeong residents and encourage them to return to the island. Analysts say the North's display of nuclear prowess and the bombardment are designed to burnish the military credentials of Kim Jong-Un, son of leader Kim Jong-Il and his heir apparent.
They say the North also appears to be pressuring the United States and other parties to restart nuclear negotiations and aid. The funeral of the two civilian victims of the North's attack was held Monday.
"I can't live. This is not what I wanted. I want to follow you," said the sobbing 83-year-old mother of one of them, clutching his portrait close to her face.