South Korea said Saturday it will go ahead with artillery drills on a border island shelled by North Korea last month despite Pyongyang's threat to retaliate again, as Russia and China expressed concerns over tension on the volatile peninsula.

The North warned on Friday it will strike even harder than before if the South went ahead with its planned drill. Four people died last month in the North's attack on Yeonpyeong Island near the tense sea border.

The U.S. supports South Korea, saying the country has a right to conduct such a military exercise. However, Russia's Foreign Ministry expressed its "extreme concern" Friday over the drills and urged South Korea to cancel them to prevent a further escalation of tension.

China, the North's key ally, also said it is firmly opposed to any acts that could worsen already-high tension on the Korean peninsula. "In regard to what could lead to worsening the situation or any escalation of acts of sabotage of regional peace and stability, China is firmly and unambiguously opposed," Chines Foreign Ministry Spokeswoman Jiang Yu said in a statement Saturday.

South Korea's Joint Chiefs of Staff said Saturday marines on the drills would push ahead with the drills as scheduled and the military is ready to respond to any possible provocation.

"We have a right to conduct our own military drills," a Joint Chiefs of Staff officer said. He declined to comment on the Russian and Chinese concerns.

The artillery drills, however, were not expected to be held over the weekend because of bad weather and would be conducted either on Monday or Tuesday, the officer said on condition of anonymity, citing office rules.

Marines carrying rifles conducted routine patrols Saturday morning on Yeonpyeong, and no warning for residents to evacuate to underground shelters has been issued.

Brief bloody naval skirmishes occurred several times along the western sea border but last month's assault was the first by the North to target a civilian area since the end of the 1950-53 Korean War. The North does not recognize the U.N.-drawn sea border in the area.

The North claims that South Korea fired artillery toward its territorial waters before it unleashed shells on the island last month, while South Korea says it launched shells southward, not toward North Korea, as part of routine exercises.

In Washington, State Department spokesman P.J. Crowley said Friday that North Korea should not view that as a threat.

"A country has every right to train and exercise its military in its own self-defense," Crowley said. "North Korea should not use any future legitimate training exercises as justification to undertake further provocative actions."

Still, Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, voiced worry of a potential chain reaction if the drill is misunderstood or if North Korea reacts negatively. "What you don't want to have happen out of that is for us to lose control of the escalation," he told reporters at the Pentagon.

A flurry of regional diplomacy was under way to defuse the tension, with New Mexico Gov. Bill Richardson visiting the North.

A frequent unofficial envoy to the reclusive country, Richardson said he wanted to visit the North's main nuclear complex and meet with senior officials during his four-day trip, though details of his schedule were unclear.

"My objective is to see if we can reduce the tension in the Korean peninsula," Richardson said at the airport in Pyongyang, according to Associated Press Television News.

U.S. Deputy Secretary of State James Steinberg held closed-door meetings Friday with Chinese State Councilor Dai Bingguo. Beijing's top foreign policy official returned last week from talks in Pyongyang with North Korean leader Kim Jong Il. China has come under growing pressure to push ally North Korea to change its behavior.

In New York, U.N. Secretary-General Ban Ki-moon called the Nov. 23 attack on the tiny island of Yeonpyeong "one of the gravest provocations since the end of Korean war."

Ban, a former South Korean foreign minister, urged North Korea to show restraint and called on both Koreas to reduce tensions on the Korean peninsula.

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AP writers Foster Klug and Kim Kwang-tae in Seoul, Young-joon Ahn in Yeonpyeong Island, South Korea; Cara Anna in Beijing and Robert Burns in Washington contributed to this report.