Officials in Seoul said they are keeping an eye out for developments in North Korea following a U.S. warning that the regime may test its nuclear or missile capabilities to mark the 110th anniversary of the birth of its late founder Kim Il-sung.

An official from the unification ministry said on Thursday that intelligence communities in the South and the U.S. are closely watching for signs of test preparations, and that the allies are in close coordination to maintain a firm defense readiness posture.

The official declined to speculate on the possibility of Pyongyang conducting a nuclear test around the birthday anniversary on April 15.

Authorities suspect that the North has begun restoring the number three shaft at its Punggye-ri nuclear testing facility in North Hamgyong Province.

U.S. Special Representative for North Korea Sung Kim on Wednesday raised the possibility of the North conducting either an intercontinental ballistic missile or nuclear test around the "Day of the Sun" on April 15.