South Korea warned the North not to make the mistake of conducting a third nuclear test and its president summoned top security officials for an unscheduled meeting on Thursday, amid signs the rival was moving ahead with preparations.
The warning by the outgoing administration of Lee Myung-bak was the toughest yet after North Korea vowed to conduct more rocket and nuclear tests in response to a U.N. censure for its launch of a long-range missile in December.
South Korea and others who have been closely observing activities at the North's known nuclear test grounds believe Pyongyang is technically ready for a nuclear test and is awaiting the final word from supreme leader Kim Jong-un.
"The government urges the North to immediately stop all acts and language of aggressive nature and abide by international obligations including U.N. Security Council resolutions," the South's presidential spokesman said.
"We warn that if the North ever makes a wrong judgment and again undertakes provocation, there will be grave consequences," the spokesman said after Lee met with security officials.
Lee called the meeting because of "specific moves" that suggested the North might be going ahead with the test, the spokesman Park Jeong-ha said.
North Korea's top state agencies have leveled fiery rhetoric since last week, also threatening to attack the South if it joined a new round of tightened sanctions under a new resolution imposed after Pyongyang's Dec. 12 rocket launch.
North Korea says the rocket launch was part of a space programme but the international community is nearly united in seeing it as a disguised test of a long-range and probably intercontinental ballistic missile.
A conservative South Korean newspaper Munhwa Ilbo later on Thursday quoted a senior government official as saying no option was off the table, when asked if military action was possible against the North.
Lee's office could not verify the nature of the comment but an official said it would not be possible to say whether a military action is or is not an option.
"North Korea must be made to feel the pain through all options to hurt it," the Munhwa daily quoted the senior government official as saying.