A Terminal High Altitude Area Defense (THAAD) interceptor is launched during a successful intercept test, in this undated handout photo provided by the U.S. Department of Defense, Missile Defense Agency.
On Wednesday, South Korea started installing key parts of a contentious United States missile defence system that also has sparked Chinese and Russian concerns.
The work to set up the Terminal High-Altitude Area Defense system, or THAAD, within this year has angered North Korea, China and Russian Federation, which see the system's powerful radars as a security threat.
"The situation prevailing on the Korean Peninsula is so tense that a nuclear war may break out due to the frantic war drills of the US imperialists and their vassal forces for aggression", Gen. Pak Yong Sik told a meeting of thousands of senior military and civilian officials.
Harris' testimony comes ahead of two congressional briefings on North Korea Wednesday afternoon, where Defense Secretary James Mattis, Joint Chiefs Chairman Gen. Joe Dunford, Secretary of State Rex Tillerson and Director of National Intelligence Dan Coats will brief the full House and Senate. On April 15 - which would have been the 105th birthday of Kim Il-sung, the country's founder - it held a large military parade, flaunting what analysts said could be new intercontinental ballistic missiles.
A US submarine created 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.
In Pyongyang's latest display, leader Kim Jong-un oversaw the country's largest-ever firing drill to mark the founding anniversary of its military, state-run media said Wednesday.
Despite the buildup, U.S. President Donald Trump has reportedly settled on a strategy that emphasizes increased pressure on North Korea with the help of China, the North's only major ally, instead of military options or trying to overthrow North Korea's government.
"Our great leaders founded and wisely led our revolutionary army, and just like that, now our respected Marshal Kim Jong Un is leading wisely, so even though the situation is tense, we are celebrating the day", said Choe Un Byol, who came with his family to the bronze statues of the former leaders.
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There have been occasions in the past when political tensions have reached crisis point - most recently in April 2013 - but Trump's firm rhetoric on North Korea in recent weeks is unusual for a U.S. leader.
Amid alarm over North Korea's expanding nuclear weapons and missile arsenal, Washington and Seoul are speeding ahead with the installment of an advanced USA anti-missile system in South Korea to cope with the growing threats. "However, we remain prepared to defend ourselves and our allies", the three top Trump administration officials said in a statement.
The USS Michigan, a nuclear-powered submarine, arrived Tuesday at the South Korean port of Busan for what was described as a routine visit to rest crew and load supplies.
"A military response would be an absolute last resort", King said. A rapid tempo of North Korean weapons testing in the past year has pushed Kim Jong Un's authoritarian nation closer to developing a nuclear-armed missile that could reach the US mainland.
A targeted US attack to take out North Korea's nuclear weapons program could spark a wider war on the Korean peninsula, lawmakers and experts have warned.
North Korea routinely accuses the United States of readying for an invasion, and threatens pre-emptive strikes to stop the U.S.
Chinese foreign ministry spokesman Geng Shuang said Wednesday that the system's deployment would "disrupt the regional strategic balance and further aggravate the tension on the peninsula".
Still, Pentagon officials have stressed to Trump there are no easy options for military interventions in North Korea.