Reuters: North Korea has placed surface-to-surface missiles on launch pads in the Yellow Sea, Yonhap news agency reported on Sunday, as the United States and South Korea began military exercises and China said it would try to ensure peace.

South Korean President Lee Myung-bak told a visiting Chinese delegation that Beijing, North Korea's only major ally which is traditionally reluctant to criticise the reclusive regime, should do more to help.

Yonhap also said North Korea had moved surface-to-air missiles to frontline areas, days after it shelled a tiny South Korean island killing four people. The North's official KCNA news agency warned of retaliatory action if its territory is violated.

"We will deliver a brutal military blow on any provocation which violates our territorial waters," KCNA said.

Officials from South Korea's Defence Ministry and the joint chiefs said they could not comment on the Yonhap report. "It is impossible to confirm the report as it is classified as military secret," an official said.

The exercises, in waters far south of the disputed maritime boundary, are being held in the face of opposition by China and threats of "consequences" from North Korea.

The chairman of North Korea's Supreme People's Assembly will visit China from Tuesday, the official Xinhua news agency said. Lee told Chinese State Councillor Dai Bingguo, who outranks Foreign Minister Yang Jiechie, that Beijing, with its growing international influence, should do more to help ensure peace.

China has not been drawn on taking sides in the conflict and declined to blame North Korea, unlike the United States, for the sinking of a South Korean naval vessel in March.

"We ask that China make a contribution to peace on the Korean peninsula by taking a more fair and responsible position on South-North Korea ties," the presidential Blue House quoted Lee as telling Dai.

"The Chinese side conveyed the message of condolences for the South Korean victims of the Yeonpyeong incident and said it would make efforts to prevent the situation from deteriorating for the sake of peace between the South and North," Lee's spokesman said.

Lee said the attack on civilians, coming after the revelation of the North's highly uranium enrichment programme, was a grave change in the situation.

"Lee asked that China play a role in North-South ties to match its growing international stature at a time when the Cold War is over and we should be pursuing coexistence and peace in the 21st century," Lee's spokesman, Hong Sang-pyo, said.

Washington says the drill is intended as a deterrent after

the worst assault on South Korea since the end of the Korean War in 1953.

Officials and journalists on the island, Yeonpyeong, were briefly evacuated to bunkers on Sunday, a Reuters witness said. The order was later withdrawn. There has been no disruption of air and shipping routes.

It also said the United States should be blamed for "orchestrating" the whole sequence of events to justify sending an aircraft carrier to join the maritime manoeuvres.

Regional giant China -- under pressure from other powers to rein in North Korea -- has said it is determined to prevent an escalation of the violence. But it warned against military acts near its coast.