S Korea-US military exercise held in the Sea of Japan
The US and South Korean militaries are taking part in a major exercise in the Sea of Japan, despite threats of retaliation from North Korea.
The four-day operation involves 20 ships, 200 planes and 8,000 personnel.
The two countries say they want to send a clear signal to the North following the sinking of a South Korean warship in March, which left 46 sailors dead.
A South Korean-led inquiry found that a North Korean torpedo sank the ship, which Pyongyang has vigorously denied.
On Saturday, North Korea threatened to use its nuclear deterrent in a retaliatory, "sacred war" in response to the exercise.
The BBC's John Sudworth, who is aboard the nuclear-powered aircraft carrier, the USS George Washington, says the show of strength is intended to rattle Pyongyang's military and political elite.
North Korea's inflammatory rhetoric is nothing new, he adds, but the rising tension is causing concern, with China urging all parties to show restraint.
The South Korean defence ministry said the manoeuvres had been relocated from the sensitive Yellow Sea to the Sea of Japan following protests from China, North Korea's ally.
Amid the rising tension, military officials in Seoul said they were closely monitoring the North's military in border areas but had not detected any unusual activity in the run-up to the exercises, code-named Invincible Spirit.
The North's National Defence Commission denounced the war games as "nothing but outright provocations aimed to stifle the Democratic People's Republic of Korea [North Korea] by force of arms," the state-run Korean Central News Agency (KCNA) said.
"The army and people of the DPRK will start a retaliatory sacred war of their own style based on nuclear deterrent any time necessary in order to counter the US imperialists and the South Korean puppet forces deliberately pushing the situation to the brink of a war," it added.
The US responded by saying it was "not interested in a war of words with North Korea".
Officers on board the USS George Washington believe the drills are a measured and restrained response to a serious act of aggression.
"We are showing our resolve," said Capt David Lausman, the carrier's commanding officer.
"North Korea may contend that it is a provocation, but I would say the opposite," he added. "It is a provocation to those who don't want peace and stability. North Korea doesn't want this."