South Korea and the United States on Thursday reaffirmed their commitment to defending "the hard-fought peace" on the divided peninsula as the allies marked the 70th anniversary of the outbreak of the Korean War.

Korean War Factfile
Factfile on the 1950-1953 Korean War. Photo: AFP / STAFF

Communist North Korea invaded the US-backed South on June 25, 1950, triggering a three-year war that killed millions.

The fighting ended with an armistice that was never replaced by a peace treaty, leaving the peninsula divided by the Demilitarized Zone and the two Koreas still technically at war.

South Korean Honour Guards
South Korean honour guards carry portraits and urns of compatriot soldiers killed during the 1950-53 Korean War, during a burial ceremony at the National Cemetery in Daejeon Photo: AFP / Jung Yeon-je

"On this day in 1950, the US-ROK military alliance was born of necessity and forged in blood," US Secretary of Defence Mark Esper and his South Korean counterpart Jeong Kyeong-doo said in a joint statement.

Korean War file photo
A young girl with her brother on her back walks past a stalled M-26 tank at Haengju, Korea in June 1951 Photo: NATIONAL ARCHIVES / NATIONAL ARCHIVES

The two paid tribute to the "sacrifice, bravery, and legacy of those who laid down their lives in defence of a free, democratic, and prosperous" South, the statement read.

Seoul's defence ministry puts the war's military fatalities at 520,000 North Koreans, 137,000 Southern troops and 37,000 Americans.

Korean War file photo
Seoul's defence ministry says 37,000 Americans troops died in the war -- this photo shows US Private First Class Edward Wilson of the 24th Infantry Regiment Photo: National Archives / Handout

The North has a different history of the conflict, which it knows as the Victorious Fatherland Liberation War, and insists that it was attacked first, before it counter-assaulted.

Korean War file photo
The fighting ended with an armistice that was never replaced by a peace treaty -- here unidentified Korean refugees rest after being forced to flee in 1951 Photo: AFP / Claude DE CHABALIER

The official Rodong Sinmun newspaper carried more than 10 stories on the war on Thursday, including an editorial asserting that a US invasion had turned "the entire country into ashes".

"A ceasefire is not peace," it said. "The enemy is aiming for the moment that we forget about June 25 and lower our guard."

South Korea
South Korea on Thursday held a ceremony to mark 70 years since the start of the Korean War. The conflict broke out on 25 June, 1950 and ended three years later - with an armistice rather than a formal peace treaty. Photo: AFPTV / Daniel DE CARTERET

The nuclear-armed North, which is subject to multiple international sanctions over its banned weapons programmes, says it needs its arsenal to deter a US invasion.

Negotiations between Pyongyang and Washington have been deadlocked for months, leaving inter-Korean relations in a deep freeze despite a rapid rapprochement in 2018 that brought three summits between North Korean leader Kim Jong Un and the South's President Moon Jae-in.

Korean War
An actor dressed as a Korean War-era soldier holds a dove during a ceremony marking the 70th anniversary of the start of the Korean War at the Baengmagoji War Memorial in Cheorwon, near the Demilitarized Zone Photo: AFP / Ed JONES

The Rodong Sinmun carried a picture of a war heroes' cemetery on the outskirts of Pyongyang, with the caption reading: "The great achievements of the victory generation will not be forgotten."

At the site of one of the key battlefields in Cheorwon county, near the Demilitarized Zone dividing the peninsula, a handful of surviving South Korean war veterans marked the anniversary.

"It is our misfortune that the South and North had to live for nearly 70 years in confrontation because of the war," a veteran said, before releasing white doves as a symbol of their hopes for a final peace settlement.

Kim on Wednesday suspended plans for military moves aimed at the South, after the North raised tensions last week by demolishing a liaison office on its side of the border that symbolised inter-Korean cooperation.

Recent events showed that inter-Korean relations "can turn into a house of cards at any time", the South's JoongAng Daily said in an editorial Thursday on the anniversary.

The South Korean government has "persistently turned a blind eye" to Pyongyang's provocations, it said, resulting in a "slackening sense of security".

"There is no free ride in keeping peace," the editorial read, adding: "We hope the government and defence ministry deeply reflect on the lesson of 70 years ago."

But the allies "remain firmly committed to defending the hard-fought peace on the Korean Peninsula," the defence ministers' statement added.

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