HONG KONG — U.S., South Korean and Japanese forces held their second day of naval exercises Thursday that have prompted threats of retaliation from North Korea, which denounced the drills as preparation for nuclear war.

U.S. sailors from the USS George Washington Strike Group are leading the exercises to practice search and rescue operations and natural-disaster response.

The two-day exercises, which were due to start on Oct. 8 and were delayed because of a typhoon, finished Thursday, Defense Ministry spokesman Wi Yong Seob said at a briefing in Seoul.

“The U.S. should know that our army is ready to confidently confront whatever turbulences and perilous provocations with powerful military forces,” the official Korean Central News Agency reported on Oct. 8, citing an unidentified Army official. In another statement on Oct. 9, KCNA called the drills “nuclear war exercises.”

North and South Korea remain divided along one of the world’s most fortified borders 60 years after the Korean War ended in a stalemate.

Tensions on the peninsula have heightened since February, when North Korea tested its third nuclear device and then threatened first strikes against the United States and South Korea over U.N. sanctions imposed after the Feb. 12 blast.

PARK'S FIRMNESS

Secretary of State John Kerry discussed North Korea with South Korean President Park Geun Hye Wednesday at the meeting of the Association of Southeast Asian Nations in Brunei. Kerry praised Park for being “firm” with the North and said that he hopes China, the North’s biggest ally, will continue to work for denuclearization.

“China always insists on realizing denuclearization and maintaining peace and stability in the Korean Peninsula, and solving the issue through dialogue and negotiations,” Foreign Ministry spokeswoman Hua Chunying said at a briefing in Beijing. "We urge all parties to do things that will help ease tensions and not the opposite.”

The exercises after a South Korean lawmaker this week confirmed that the North has restarted its Yongbyon nuclear facility, much of which had been mothballed under an agreement in 2007 as a result of talks that aimed to end its nuclear program in return for aid.

In 2009, North Korea formally pulled out of the talks, which included the South, the United States, Russia, Japan, and China, and months later held its second nuclear test.

PLUTONIUM PRODUCTION

Yongbyon, the country’s main nuclear facility, is capable of producing enough plutonium to make one bomb a year, according to the U.S.-Korea Institute at Johns Hopkins University’s School of Advanced International Studies, which reported on Sept. 12 that the plant had restarted, based on satellite imagery.

North Korea followed its threats of first strikes in March with an announcement on April 2 that it would restart all facilities at Yongbyon to produce energy and bolster its “nuclear armed force both in quality and quantity.”

The naval exercises are being held a little more than a week after the U.S. and South Korea signed a new joint-defense strategy that would allow for first strikes against North Korea to counter any possible threat of nuclear attack.

The agreement affirmed a U.S. commitment to use “the full range of military capabilities” to deter North Korea, the two governments said in a communique after the Oct. 2 signing.

LEADERSHIP CHANGES

The exercises also coincide with signs that North Korean leader Kim Jong Un has replaced his army chief of staff for the third time since taking over the country and its 1.2-million strong military less than two years ago.

KCNA referred to Ri Yong Gil Wednesday as Chief of General Staff of the Korean People’s Army in a report about a public appearance by Kim, indicating Kim Kyok Sik no longer holds the post.

Ri, who reports say is in his 60s, is believed to have been named to the post in August, when Kim’s name and image stopped appearing in the North Korean press. Kim, who has been described as a hardliner, led the military unit that South Korea suspects of carrying out two attacks in 2010.

South Korea said the North sunk one of its patrol ships in March 2010, killing 46 sailors. In November of that year, North Korea lobbed artillery shells at Yeonpyeong Island near the border, killing two marines and prompting South Korea to return fire and mobilize fighter jets.