South Korea ? On the 57th anniversary of the armistice that ended the Korean War, U.S. and South Korean ships intensified high-profile military exercises Tuesday that underscore rising tensions in a region yet to truly find peace.

The massive maneuvers, called “Invincible Spirit,” are being conducted by an armada of South Korean and U.S. ships — including the USS George Washington supercarrier — in international waters off the South Korean coast.

The normally quiet patch of the East Sea has been a buzz of military activity — with helicopters dropping sonar buoys into the waters, squadrons of carrier-based F-18 fighters embarking on bombing runs and destroyers blasting their guns at unmanned aerial drones.

The exercises come just four months after a South Korean warship was sunk, allegedly by a torpedo from a North Korean submarine, in the worst attack on the South’s military since the Korean War ended in a shaky truce on July 27, 1953.

North Korea has denied attacking the ship in which 46 sailors died. It has threatened to retaliate over the four-day maneuvers that end today, but senior officers aboard the U.S. ships said no North Korean military activity has been observed.