South Korean President Moon Jae In on Wednesday ordered the nation's military and government officials to make "thorough preparations" to counter any possible North Korean provocations.

Moon made the remarks during a visit to a wartime command headquarters for Ulchi-Freedom Guardian, an annual joint exercise with the United States, his spokesman Park Soo Hyun said during a press briefing.

"Through the exercise, the central government and local governments should closely cooperate to provide support in ensuring South Korea-U.S. combined forces can focus just on (military) operations," Moon was quoted as saying.

During his visit, Moon was briefed on the current military situation on the Korean Peninsula and he thanked the troops for their efforts to maintain defense preparedness, Park said.

This year's 10-day joint military exercise with the United States began on Monday. On the same day, South Korea also launched a four-day defense exercise involving 480,000 officials from 4,000 cities, counties and regional districts to prepare for contingencies and protect the public.

Meanwhile, an annual nationwide air-raid drill was held Wednesday in South Korea, aimed at training people to take shelter during a possible air raid by North Korea.

When the sirens rang at 2 p.m. for the 20-minute drill, pedestrians were asked to find the nearest underground shelter or passage at basement level.

Tension on the Korean Peninsula remains high after North Korea warned the United States that it could face "merciless revenge" for ignoring Pyongyang's warnings not to go ahead with the annual joint military exercise with South Korea.

North Korea was responding to the start Monday of the computer-simulation command post exercise, which involves tens of thousands of U.S. and South Korean troops and comes after the North successfully tested two intercontinental ballistic missiles last month.

While the United States and South Korea claim the joint exercise is entirely defensive, Pyongyang views it as a rehearsal for an invasion of the North.

South Korea and the United States technically remain still at war with North Korea, as the 1950-1953 Korean War ended with an armistice rather than a permanent peace treaty.