美韩开始年度联合军演，朝鲜反应强烈

South Korea and the United States have begun their annual joint military exercise under the threat of military retaliation by North Korea.

About 25,000 U.S. forces and 50,000 South Korean troops are involved in the two-week Operation Ulchi Freedom exercise, which is largely simulated. The annual drills routinely anger Pyongyang, which says it sees the drills as a rehearsal for a full-scale invasion of the North, despite insistence by Seoul and Washington that the drills are purely defensive in nature.

A statement issued Monday by the North's military said its first-strike units were ready to turn the U.S. and Washington "into a heap of ashes through a Korean-style preemptive nuclear strike," if North Korea's sovereignty is threatened.

Tensions on the Korean peninsula have worsened in recent months, with North Korea under harsh U.N. sanctions over a series of tests of its nuclear weapons and ballistic missiles. Relations are also likely aggravated by the recent defection of a high-ranking diplomat, Thae Yong Ho; Pyongyang's deputy ambassador to Britain. The North denounced Thae as "human scum" and accused him of a number of criminal acts, including child molestation.

South Korean President Park Guen-hye told her National Security Council Monday that Thae's defection was a sign of "serious cracks" in North Korea's ruling elites.