South Korea warned Tuesday that North Korea may be preparing to conduct a fourth nuclear test, citing heightened activity monitored at the North's main test site, days ahead of US President Barack Obama's visit to Seoul on Friday.

"Our military is currently detecting a lot of activity in and around the Punggye-ri nuclear test site," South Korea's Defense Ministry spokesman Kim Min-seok told reporters.

North Korea conducted three previous nuclear tests at this site in 2006, 2009 and last year, all of which incurred the imposition of UN sanctions.

Kim noted that North Korea appears to be ready to carry out a nuclear test at any time if it wishes. The nation has recently made remarks claiming to be preparing something "unimaginable" for its enemies and "something big" before April 30, according to South Korea's Yonhap News Agency.

North Korea's foreign ministry warned on March 30 that it would not "rule out a new form of nuclear test" after the UN Security Council condemned Pyongyang's firing of two Rodong medium-range ballistic missiles on March 26 in violation of UN resolutions.

Nuclear expert Jeffrey Lewis at the Monterey Institute of International Studies in the US said the new form of nuclear test could mean a simultaneous detonation of two or more devices, Reuters reported.

Kim did not specify the monitored activity, but cautioned that this may be a decoy by the North to raise tensions ahead of Obama's visit.

Obama will visit Seoul on Friday following a two-day trip to Japan, and North Korea is expected to be high on his agenda.

"We are thinking it is possible that the North may stage a surprise nuclear test or just pretend to stage a nuclear test," Kim said.

Analysts are skeptical of a nuclear test at the current time.

"The activity around the nuclear test site is probably a masquerade to get attention from the US government, which has been indifferent to North Korea," Ahn Chan-il, a former defector who heads the World Institute for North Korea Studies in Seoul, told the Global Times.

Ahn said the possibility of a new nuclear test is slim "because it will turn China against the North Korea regime."

Da Zhigang, director of the Institute of Northeast Asian Studies at Heilongjiang Provincial Academy of Social Sciences, said North Korea is highly likely to carry out a fourth nuclear test, but this would not happen during Obama's visit.

"Pyongyang will wait to decide whether to conduct a fourth test based on its observation of what comes out of Obama's visit to the two allies of the US. It knows that a nuclear test at the current time will only push the three countries to be more unified against North Korea," he told the Global Times.

If there is a new nuclear test, it will possibly be conducted in May or June, Da added.

Pyongyang warned Monday that Obama's visit will only serve to escalate confrontation and "bring dark clouds of a nuclear arms race" over the Korean Peninsula.

Chinese foreign ministry spokesperson Qin Gang on Tuesday called on relevant parties to remain calm and restrained and adhere to the goal of realizing a nuclear-free Korean Peninsula.

Wu Dawei, China's Special Representative on the Korean Peninsula Affairs, has described North Korea's threats of a nuclear test as "serious" and noted that Beijing is making diplomatic efforts to prevent Pyongyang from carrying out its fourth nuclear test, Yonhap reported Tuesday, citing a diplomatic source with knowledge of the matter.

Considering the sanctions imposed by the UN and China's stance, North Korea will be cautious in conducting the new test, but there are no signs indicating Beijing is able to make Pyongyang give up nuclear weapons, Da said.

South Korean Foreign Minister Yun Byung-se said Monday that another nuclear test by the North will be a "game changer," warning Pyongyang that it was playing an "unwinnable game."