The US military has begun moving parts of the controversial THAAD anti-missile defence system to a deployment site in South Korea amid high tensions over North Korea's missile and nuclear programs.
Washington and Pyongyang have been ratcheting up pressure on each other in recent weeks, with the United States sending a aircraft carrier group and nuclear submarine to the region and North Korea attempting more missile launches in defiance of layers of UN sanctions.
North Korea said on Wednesday leader Kim Jong Un had supervised the country's "largest-ever" live-fire drill to mark the 85th founding anniversary of its military, with more than 300 large-calibre, self-propelled artillery guns demonstrating their fire power at an event on its east coast.
The firing drill came instead of a nuclear test or the launch of a long-range missile as feared amid pressure from the United States and China, its sole major ally, which has been irritated by Pyongyang's weapons development.
The United States and South Korea agreed to deploy the Terminal High Altitude Area Defence (THAAD) system in response to the threat of missile launches by North Korea, however the move has angered China, which says the advanced system will do little to deter the North while destabilising the regional security balance.
South Korea's defence ministry said some elements of THAAD were moved to the site on what had been a golf course in the county of Seongju in the country's southern region.
"South Korea and the United States have been working to secure an early operational capability of the THAAD system in response to North Korea's advancing nuclear and missile threat," the ministry said in a statement.
A US submarine designed to carry 150 Tomahawk cruise missiles entered a South Korean port on Tuesday as the USS Carl Vinson carrier group steamed towards the Korean waters in an effort to deter the North from a sixth nuclear test and more missile launches.
South Korea's navy has said it plans to hold a joint drill with the US strike group late this month.
The spokesman for the front runner in South Korea's May 9 presidential election Moon Jae-in said the move by the US military to deploy elements of the THAAD anti-missile defense was strongly regrettable.
Moon's spokesman, Park Kwang-on, said in a statement that the move was "very inappropriate" as it strips the next government of the right to make the policy decision on the controversial missile defense system.