North Korea threatened military action against South Korea on Sunday, one day before the South Korea-U.S. "Key Resolve" joint military exercise was set to begin.

North Korean representatives at the inter-Korean border village of Panmunjeom said in a statement that the North will, if provoked, strike back and turn the South Korean capital Seoul into a "sea of fire."

Authorities in Seoul say the threat may be a North Korean tactic to create a cause to pursue provocations during the Key Resolve exercise period.

The South Korean military has strengthened its surveillance of the North Korean military near the Military Demarcation Line and the maritime Northern Limit Line.