North Korea ratcheted up its criticism of the ongoing joint military drills between South Korea and the United States Wednesday, calling them "an anti-unification sin" and threatening "merciless punishment."

Seoul and Washington kicked off their annual Key Resolve and Foal Eagle military exercises on Monday as part of efforts to improve the combined forces' operations and combat capabilities to deter threats from the communist country.

The North routinely denounces the annual drills as a rehearsal for invasion of the regime, although the allies say they are only defensive in nature.

"If warmongers at home and from abroad trigger war on this soil, (we) will turn the bases of their provocations into ruins of ashes and into a pitiful grave," the Rodong Sinmun, the main organ of the ruling Workers' Party of Korea, said in an article.

Labeling the joint Seoul-Washington drills a war simulation targeting North Korea, the newspaper denounced the drills as an anti-unification crime that impairs the possibility of improving inter-Korean relations.

The newspaper also took issue with the deployment of the USS Fort Worth in the drills. It claimed the participation of the 3,450-ton U.S. littoral combat ship is clear evidence the drills aim for an actual attack rather than just defensive purposes.

"If one drop of provocative sparks lands where our sovereignty is exercised, (North Korea) will immediately hit back," the newspaper said in a separate article.

On Monday, Pyongyang fired two short-range missiles into the East Sea in an apparent protest against the start of the military drills. (Yonhap)