President Lee Myung-bak has ordered government agencies to be on the alert for a possible nuclear test by North Korea, his spokesman said Sunday, as neighboring countries put pressure on Pyongyang to scrap its plan.

Lee made an unannounced visit Sunday to the underground bunker of the presidential compound Cheong Wa Dae, which serves as the national crisis management center, said Park Jung-ha, the president's spokesman.

Lee called on all government offices to keep a close tab on North Korea as it is set to carry out its third nuclear test and urged them to be prepared for contingencies, the spokesman said.

Lee was briefed by Chun Yung-woo, the senior presidential security secretary, and Ahn Kwang-chan, the head of Cheong Wa Dae's office for national crisis management, on the latest indications of a North Korean nuclear test and other developments in the North Korean military, according to Park.

Last Friday, South Korean government sources said the North has placed a screen over the entrance to the western tunnel in its test site in Punggye-ri in the North's northeastern tip. This was seen as an attempt to block outside monitoring of the detonation.

The North has threatened to conduct its third nuclear test, following earlier tests in 2006 and 2009, in retaliation for the U.N. Security Council's resolution condemning its Dec. 12 long-range rocket launch.

As speculation runs rampant that a nuclear test is imminent, Lim Sung-nam, South Korea's chief nuclear envoy, will fly to China later Sunday for consultations with his Chinese counterpart, Wu Dawei, according to a South Korean foreign ministry official.

"Lim will have talks on North Korean and nuclear matters in China," the official said, adding that Lim is expected to return home on Tuesday.

Diplomatic sources said Lim and Wu will meet in Beijing on Monday to discuss measures to deter North Korea from carrying out further provocations.

Lim is expected to stress that denuclearization of the Korean Peninsula is a prerequisite for peace and stability, and that a third North Korean nuclear test will put security of the rest of the region in danger.

Lim could also talk about the need for a stern response from the international community to another nuclear test by the North.

Lee Kyu-hyung, the South Korean ambassador to Beijing, has already met with Wu and other senior Chinese diplomats, and asked them to help deter North Korea from carrying out the nuclear test, sources in Beijing said Sunday.

China has done its part to pressure North Korea. Multiple diplomatic sources in Beijing said the Chinese foreign ministry has summoned Ji Jae-ryong, the North Korean ambassador to China, on multiple occasions since Pyongyang declared its intentions on Jan. 24 to conduct the nuclear test.

China expressed concerns that North Korea has abandoned its denuclearization efforts and urged Pyongyang to "hold off" on plans for the nuclear test, the sources said, adding the Chinese officials also asked the North Koreans to "make the right decision," since its third nuclear test would generate an unprecedented amount of tension on the Korean Peninsula.

China has decided not to make these summons public, given its close ties with North Korea, the sources said.

Despite rumors that China could dispatch a high-level special envoy to Pyongyang to prevent the nuclear test, there has been no indication that high-ranking officials from either side of the border have made the trip. (Yonhap News)