South Korea's presidential office Cheong Wa Dae is keeping close tabs on the latest North Korean developments amid speculation that the reclusive country is close to conducting its sixth nuclear test, officials said Sunday.

"We're closely observing the possibility of provocations," a Cheong Wa Dae official told Yonhap News Agency over the phone, adding that the office remains vigilant, assessing that the North could carry out a provocation at anytime.

Despite the ousting of President Park Geun-hye earlier this month over a political scandal, Cheon Wa Dae's National Security Office is reportedly on emergency duty, monitoring developments in North Korea and reviewing countermeasure scenarios the South may take in the case of a real nuclear test.

Experts have speculated on several time windows for a possible nuclear test: either during the US-China summit from April 6-7, the North's Supreme People's Assembly on April 12 or on the 105th anniversary of founder Kim Il-sung's birthday, which falls on April 15.

If a nuclear test indeed takes place, Acting President and Prime Minister Hwang Kyo-ahn will likely convene a National Security Council to evaluate the North's latest provocation and mull countermeasures.

Pyongyang carried out two underground nuclear tests last year alone, following those in 2006, 2009 and 2013.

The country's leader said in his New Year's message that the North has entered the final stage of preparing to launch an intercontinental ballistic missile, which is aimed at hitting the US mainland. (Yonhap)