Seoul:
North Korea's army threatened an immediate retaliatory strike should a "single shell" fall on its side of the disputed maritime border during scheduled South Korean-US military drills.
Any subsequent counterstrike would trigger an escalated military reaction that would see South Korea's border islands engulfed in a "sea of flames," said a statement from the southwestern command of the Korean People's Army.
South Korean live-artillery exercises near the Yellow Sea border were an attempt to tip prevailing military tensions into an "actual war," said the statement carried by the official Korean Central News Agency.
North Korean troops in the sector have been ordered to take "immediate counteractions in case even a single shells drops" across the border, it said.
South Korea and the United States are reportedly staging a five-day joint anti-submarine drill until Friday near the maritime border, involving a US nuclear-powered attack submarine, the USS Bremerton, and a number of warships.
"In case the enemies recklessly counter our counterstrikes, all striking forces will turn the (South's) five islands ... into a sea of flames," the army statement said.
The five islands lie near the disputed border that was the scene of bloody naval battles in 1999, 2002 and 2009.
One of them - Yeonpyeong - was shelled by the North in November 2010, killing four South Koreans, including two civilians, and sparking brief fears of a full-scale conflict.
South Korea and the United States recently wrapped up two large-scale, annual joint military drills.
Tensions on the Korean peninsula have been high since the North, angered by the joint exercises and by UN sanctions imposed for its February nuclear test, issued a series of nuclear threats against Seoul and Washington.