SEOUL, South Korea – North Korea's military warned Tuesday that its artillery and rocket forces are at their highest-level combat posture — the latest in a string of bellicose threats aimed at South Korea and the United States.

The announcement came as South Koreans marked the third anniversary of the sinking of a warship in which 46 South Korean sailors died. Seoul says the ship was hit by a North Korean torpedo; the North denies involvement.

South Korean President Park Geun-hye paid her respects to the sailors and, in a nationally televised speech, warned that "North Korea must immediately abandon its thought that nuclear weapons will protect its regime."

Seoul's Defense Ministry said Tuesday it hasn't seen any suspicious North Korean military activity and that officials are analyzing the North's warning. Analysts say a direct North Korean attack is extremely unlikely, especially during joint U.S.-South Korean military drills that end April 30, but there's some worry about a provocation after the training wraps up.

North Korea, angry over routine U.S.-South Korean drills and recent U.N. sanctions punishing it for its Feb. 12 nuclear test, has vowed to launch a nuclear strike against the United States and repeated its nearly 20-year-old threat to reduce Seoul to a "sea of fire." Despite the rhetoric, outside weapons analysts have seen no proof that North Korea has mastered the technology to build a warhead small enough to mount on a missile.

The North Korean army's Supreme Command said Tuesday it will take "practical military action" to protect national sovereignty and its leadership in response to what it called U.S. and South Korean plots to attack.

The statement, carried by the North's official Korean Central News Agency, cited the participation of nuclear-capable B-52 bombers in South Korea-U.S. drills.

North Korea's field artillery forces — including strategic rocket and long-range artillery units that are "assigned to strike bases of the U.S. imperialist aggressor troops in the U.S. mainland and on Hawaii and Guam and other operational zones in the Pacific as well as all the enemy targets in South Korea and its vicinity" — will be placed on "the highest alert from this moment," the statement said.

The North's recent threats are seen partly as efforts to strengthen internal loyalty to young leader Kim Jong Un.

The New York Times contributed to this report.