Seoul warned North Korea Thursday that it would face grave consequences if it presses ahead with an envisioned nuclear test.
The warning came following an emergency meeting of President Lee Myung-bak and foreign affairs and security-related ministers at Cheong Wa Dae.
There have been increasing signs that the North is putting the final touches on preparations for the seemingly imminent nuclear test.
"Seoul urges Pyongyang to stop any provocative acts and instead fulfill its international obligations, including abiding by UNSC resolutions," the government said after the meeting. "If Pyongyang, driven by miscalculation, conducts another provocative act, we warn that it will face serious results."
President Lee instructed the Ministry of National Defense to prepare strong countermeasures against any provocative action by North Korea.
The defense ministry upgraded its readiness posture to the second highest level. It said forces stationed near the military demarcation line and the maritime border have been ordered to be prepared for possible North Korean attacks and to respond immediately to them.
Lee called on the foreign ministry to work closely with permanent members of the U.N. Security Council as South Korea will be the rotating chair for the month of February.
A Seoul official told reporters Thursday on condition of anonymity that a third nuclear test could occur any time if North Korean leader Kim Jong-un "gives the green light to it," noting that the North is seen as having completed all necessary preparations.
The official said "domestic demand" appeared to be prompting Kim to play the high-stakes poker game.
"The North might have miscalculated that it has got nothing to lose even if it goes ahead with a third nuclear test because it has survived tight, multilayered international sanctions following the first and second nuclear tests," he said.
The official said Seoul is closely cooperating with allies to thwart the North Korean threat and that they are keeping all options open.
He declined to give further details about what those options could be.
Last week, the UNSC adopted a fresh resolution condemning the North for its launch of the long-range Unha-3 rocket which allegedly put a satellite into orbit in December.
It tightened measures against the Stalinist state by subjecting more North Korean firms and individuals responsible for the proliferation of weapons of mass destruction to existing sanctions.