YEONPYEONG, South Korea (Reuters) - South Korea’s defense minister resigned on Thursday, two days after an attack by North Korea and amid criticism that the South’s response was too slow.

Kim Tae-young became the first political victim of the attack as China expressed muted criticism of forthcoming joint U.S-south Korean military exercises in the Yellow Sea.

President Lee Myung-bak accepting his minister’s resignation “to improve the atmosphere in the military and to handle the series of incidents”, a presidential official said.

Kim had also tended his resignation in May after the sinking of a South Korean naval vessel in March, but Lee asked him to stay on in the job. The Cheonan attack, in which 46 sailors were killed, was also blamed on the North.

North Korea fired a barrage of artillery shells at the island of Yeonpyeong off the peninsula’s west coast on Tuesday, killing two civilians and two soldiers and destroying dozens of houses.

South Korean troops fired back 13 minutes later, causing unknown damage. Members of Lee’s own party and opposition lawmakers accused the military of responding too slowly.

The government was also criticized for its perceived weak response to the Cheonan incident. North Korea has denied responsibility for that attack.

While China objected to the joint military exercises starting this weekend, North Korea threatened further attacks on the South if there were more “provocations”.

Seoul said it would increase troops on islands near North Korea after the bombardment, which caused a sharp spike in tension in the world’s fastest growing region.

Washington is putting increasing pressure on China to rein in North Korea, but a foreign ministry spokesman in Beijing said what was needed was a revival of the stalled six-party talks involving the two Koreas, Russia, China, Japan and the United States.

“We have noted the relevant reports and express our concern about this,” spokesman Hong Lei said, referring to the joint military exercises and the involvement of the nuclear-powered aircraft carrier the USS George Washington in the drill.

But Beijing has previously used stronger language to signal its displeasure. In August, the People’s Liberation Army said earlier plans to send the George Washington to the Yellow Sea would make it lose respect and threatened long-term damage to Sino-U.S. relations.

Seoul expressed frustration with Beijing for not taking sides, noting even Russia had condemned this week’s attack.

“We must engage with China for it to take more responsibility on North Korea’s behavior,” said a government official, who asked not to be identified.

China has long propped up the Pyongyang leadership, worried that a collapse of the North could bring instability to its own borders. Beijing is also wary of a unified Korea that would be dominated by the United States, the key ally of the South.

BELLICOSE RHETORIC

There was no let-up in the typically bellicose language used by North Korea.

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“(North Korea) will wage second and even third rounds of attacks without any hesitation if warmongers in South Korea make reckless military provocations again,” the North’s KCNA news agency quoted a statement from the military as saying.

“The U.S. cannot evade the blame for the recent shelling,” it added. “If the U.S. truly desires detente on the Korean peninsula, it should not thoughtlessly shelter the South Korean puppet forces, but strictly control them so that they may not commit any more adventurous military provocations.”

North Korea said the shelling was in self-defense after Seoul fired shells into its waters. The South Korean government official said Seoul had been shocked by the attack for its “indiscriminate bombing of civilians”.

“We could not imagine such a grave provocation,” he said. “But now we see any kind of provocation is possible ... and it makes it very easy to respond in the future.”

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The official said Tuesday’s artillery attack on Yeonpyeong island could only have been ordered by reclusive North Korean leader Kim Jong-il.

Kim and his son and designated heir, Jong-un, visited the Yellow Sea coastal artillery base from where shells were fired at Yeonpyeong just hours before the attack, North Korean media reported.

“This kind of serious provocation could only be planned by Kim Jong-il,” the official said, adding the attack was also designed to promote the younger Kim’s military credentials.

U.S. officials have said the attack appeared linked to the upcoming succession in North Korea’s leadership.

It was the heaviest attack by the North since the Korean War ended in 1953 and marked the first civilian deaths in an assault since the bombing of a South Korean airliner in 1987.

A bitterly cold wind was blowing through the hilly island on Thursday when reporters were allowed to visit for the first time since the attack. North Korea was clearly visible.

Houses were deserted with most residents having fled to the mainland, many with their roofs caved in and charred black. Broken glass was strewn in the narrow alleyways.

The deaths of civilians have added to anger in the South, and sparked heated debate in parliament over the military’s slow response -- as well as calls for heads to roll. Defense Minister Kim had been one of the main targets.

While the rhetoric continues, global markets have moved on to other issues after Tuesday’s attack. The stock market opened up in Seoul on Thursday but closed almost flat. (Reporting by Seoul bureau; Writing by Jeremy Laurence; Editing by Nick Macfie and Ron Popeski)