South Korea and the U.S. have forged an upgraded operational plan that will pave the way for a preemptive strike such as in a contingency case involving the use of weapons of mass destruction, military sources said Thursday.

The OPLAN 5015 was formulated last June to replace OPLAN 5027 by the chairmen of the two countries’ Joint Chiefs of Staff. The allies have, since 1974, been assessing their joint defense plan every year, primarily to devise better responses to North Korea’s military threats.

“It is the first time that the OPLAN has dealt with the concept of a preemptive strike,” a military source said, requesting anonymity due to the sensitivity of the matter.

The updated scheme, which has already come into force, will allow the allies to consider a preemptive strike aimed at terminating WMDs such as atomic bombs, missiles and biological and chemical weapons when signs of their imminent use are detected from the North.

They are believed to have been carrying out the joint Ulchi Guardian Freedom exercise under the scenario, focusing on minimizing damage in case of Pyongyang’s use of a biochemical weapon.

“The plan was revised as part of the two countries’ annual efforts to reflect the threats we face, and they usually examine if there is any need to adjust their strategies after the UFG ends every year,” another official told The Korea Herald on condition of anonymity.

Seoul’s Defense Ministry declined to confirm the report, citing classified operational strategies. “Any revelation of our military’s operational plan does not contribute to not only our security, but also national interest,” it said in a statement.

The discovery came after cross-border tensions flared to the verge of an armed conflict, with the two Koreas trading fire and the North declaring a “quasi-state of war” and putting frontline forces in full combat readiness.

But Defense Minister Han Min-koo sought to decouple the latest crisis and the OPLAN update, noting that any change in the war plan could reflect the U.S.’ overall military cutbacks in the face of its budget woes and resulting the sequestration.

“The two countries update their joint defense plan every year and maintain close coordination at all times,” he said during parliamentary questioning Tuesday.

“There have been calls for an OPLAN review given the changing situation on the U.S. part including the Army scale back and the sequester, but the North’s semi-war state declaration and any immediate OPLAN amendment are separate issues.”

By Shin Hyon-hee (heeshin@heraldcorp.com)