This undated computer graphic image shows North Korea's national flag, the scene of its nuclear test and a satellite image of its nuclear test site in Punggye-ri. (Yonhap)

SEOUL, July 18 (Yonhap) — South Korea’s Ministry of Unification said on Monday reported that North Korea could conduct its fifth nuclear test whenever it wants, amid growing speculations that the reclusive country may be planning another detonation following increased activity detected at its underground nuclear site.

“North Korea seems to be fully prepared to carry out a nuclear test at any time (when the order is given by its leader Kim Jong-un). South Korea is closely monitoring every move at the North’s test site in close cooperation with the United States,” unification ministry spokesman Jeong Joon-hee said at a regular press briefing.

South Korean and U.S. intelligence sources said Sunday they had detected a spike in movement of vehicles and personnel at the North’s nuclear test site in Punggye-ri, in the northwestern part of North Korea. The rise in movement comes after Seoul and Washington on July 8 announced their plan to deploy the Terminal High-Altitude Area Defense (THAAD) system in South Korea by 2017.

The ministry didn’t provide any further details on the North’s preparations for a possible nuclear test.

Early on Monday, Japan’s Kyodo News said that signs have been spotted of North Korea preparing to carry out a nuclear test as early as this month, citing government officials from South Korea and Japan.

North Korea conducted nuclear tests in 2006, 2009 and 2013, drawing international condemnation and U.N. sanctions. It conducted its fourth test in January of this year, that caused the United Nations to slap the toughest ever sanctions to date on the country.