The United States stressed sanctions over diplomacy to rein in North Korea, brushing aside China's call for talks as US and South Korean warships staged a show of strength Tuesday against Pyongyang.

The North's deadly artillery attack on a South Korean border island last week has highlighted deep differences between Beijing and Washington and its allies over how to restrain the North.

China's refusal publicly to condemn its ally for the shelling -- the first of a civilian area in the South since the 1950-53 war -- has sparked anger and public protests in South Korea.

The United States and Japan, meanwhile, have been dismissive about China's call for emergency consultations on the crisis between envoys to stalled six-nation talks on the North's nuclear disarmament.

The White House said such talks would amount to "PR activity" unless Pyongyang changes its behaviour.

"The North Koreans need to demonstrate a seriousness of purpose in ending their aggressive behaviour," spokesman Robert Gibbs said Monday.

Gibbs said the United States and other countries "were not interested in stabilising the region through a series of PR (public relations) activities".

US ambassador Susan Rice separately called for tighter enforcement of UN sanctions in response to the "outrageous" artillery attack. China should play a "responsible leadership role" in defusing the crisis, she said.

Japan's foreign minister also faulted China's proposal.

"It's unacceptable for us to hold six-party talks only because North Korea has gone amok," Seiji Maehara told the Wall Street Journal.

"We must first see some kind of sincere effort from North Korea, on its uranium enrichment programme and the latest incident."

The North's attack on Yeonpyeong island, which killed two civilians and two marines and injured 18 other people, came days after the disclosure that it has an apparently functioning uranium enrichment plant.

Analysts say the two events appear designed to bolster the military credentials of leader-in-waiting Kim Jong-Un, or to pressure the United States and South Korea into holding dialogue and possibly resuming aid.

The North says the plant will produce fuel for electricity generation but experts and senior US officials say it could easily be configured to make weapons-grade uranium.

The North's state media said Tuesday "many thousands of centrifuges" are operating at the plant, claiming it is for peaceful purposes. The country has already tested two bombs made from plutonium.

South Korea is strengthening artillery and troop numbers on its islands near the disputed Yellow Sea border in response to the attack.

President Lee Myung-Bak, in a toughly-worded speech Monday, did not mention China's talks proposal but said the North would not voluntarily mend its ways.

"If the North commits any additional provocations against the South, we will make sure that it pays a dear price without fail," he said, calling the shelling "a crime against humanity".

Far to the south of the border, 11 US and South Korean warships headed by the US carrier George Washington were staging a third day of exercises aimed at sending a message of deterrence to the North.

Tuesday's drills were practising defence against the intrusion of North Korean ships and against air attacks, the South's military said.

The western sea border has been an enduring flashpoint, with deadly naval clashes there in 1999, 2002 and last November. In March a South Korean warship sank there, after what Seoul says was a torpedo attack by Pyongyang.

The North says the "Northern Limit Line", drawn by United Nations forces after the war, should run further south.

"The NLL on which South Korean war addicts are insisting is an illegal and non-existent ghost line," said Rodong Sinmun, the ruling communist party newspaper.

"If South Koreans do not learn lessons from this (shelling) incident and insist the NLL is their borderline, then we will take strong reactive measures."

State media also blasted the current naval drills, calling them provocative and claiming they heighten the risk of war.

"We have full deterrence to destroy our enemies at once," said cabinet newspaper Minju Chosun. "If the US and South Korean enemies dare to fire one shell in our territory and sea territory, they will have to pay for it."