North Korea has condemned the start of US-South Korean military drills with fiery rhetoric which was given added weight by recent missile and nuclear tests by Pyongyang.
Annual exercises between Seoul and Washington always rile the North, which calls the defensive drills a rehearsal for an invasion.
The North will "mercilessly foil the nuclear war racket of the aggressors with its treasured nuclear sword", said a spokesman for the General Staff of the Korean People's Army.
He said the North's reaction will be the toughest ever, but did not elaborate.
There is renewed worry in Seoul and Washington because North Korea test-launched a new intermediate-range missile in February, after two nuclear tests last year.
South Korea's military said it will "resolutely" punish North Korea if provoked.
The drills are field training that will run until late April. More training involving computer-simulated war games will start in the coming weeks.
The US has 28,500 troops stationed in South Korea as a deterrent against potential aggression from North Korea.
The Korean peninsula remains in a technical state of war because the 1950-53 Korean War ended with an armistice, not a peace treaty.