Seoul, South Korea (CNN) A North Korean soldier defected to South Korea on Monday across the heavily fortified border that separates the two countries, South Korean authorities said.

The soldier crossed the Military Demarcation Line on foot from the eastern North Korean province of Gangwon, said a South Korean Defense Ministry official, who declined to be identified.

A North Korean military honor guard stands at attention at Pyongyang's airport in May 2001.

A North Korean military honor guard stands at attention at Pyongyang's airport in May 2001.

A North Korean controller is seen along the railway line between the Pyongyang and North Pyongan provinces in April 2012.

A North Korean controller is seen along the railway line between the Pyongyang and North Pyongan provinces in April 2012.

Members of a North Korean military band gather following an official ceremony at the Kim Il Sung stadium in Pyongyang in April 2012.

Members of a North Korean military band gather following an official ceremony at the Kim Il Sung stadium in Pyongyang in April 2012.

North Korean soldiers listen to a speech during an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang in April 2012.

North Korean soldiers listen to a speech during an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang in April 2012.

North Korean performers sit below a screen showing images of leader Kim Jong Un in Pyongyang in April 2012.

North Korean performers sit below a screen showing images of leader Kim Jong Un in Pyongyang in April 2012.

Kim Jong Un applauds as he watches a military parade in Pyongyang in April 2012.

Kim Jong Un applauds as he watches a military parade in Pyongyang in April 2012.

North Korean soldiers relax at the end of an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang in April 2012.

North Korean soldiers relax at the end of an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang in April 2012.

A closer look at the UNHA III rocket on its launch pad in Tang Chung Ri, North Korea.

– A closer look at the UNHA III rocket on its launch pad in Tang Chung Ri, North Korea.

In April 2012, Pyongyang launched a long-range rocket that broke apart and fell into the sea. Here, the UNHA III rocket is pictured on its launch pad in Tang Chung Ri, North Korea.

In April 2012, Pyongyang launched a long-range rocket that broke apart and fell into the sea. Here, the UNHA III rocket is pictured on its launch pad in Tang Chung Ri, North Korea.

A North Korean soldier stands guard in front of an UNHA III rocket at the Tangachai-ri Space Center in April 2012.

A North Korean soldier stands guard in front of an UNHA III rocket at the Tangachai-ri Space Center in April 2012.

A crowd watches as statues of North Korean founder Kim Il Sung and his son Kim Jong Il are unveiled during a ceremony in Pyongyang in April 2012.

A crowd watches as statues of North Korean founder Kim Il Sung and his son Kim Jong Il are unveiled during a ceremony in Pyongyang in April 2012.

Kim visits the Rungna People's Pleasure Ground, under construction in Pyongyang, in a photo released in July 2012 by the KCNA.

Kim visits the Rungna People's Pleasure Ground, under construction in Pyongyang, in a photo released in July 2012 by the KCNA.

Kim, center, poses in this undated picture released by North Korea's official news agency in November 2012.

Kim, center, poses in this undated picture released by North Korea's official news agency in November 2012.

In a photo released by the official North Korean news agency in December 2012, Kim celebrates a rocket's launch with staff from the satellite control center in Pyongyang.

In a photo released by the official North Korean news agency in December 2012, Kim celebrates a rocket's launch with staff from the satellite control center in Pyongyang.

Soldiers in the North Korean army train at an undisclosed location in March 2013.

Soldiers in the North Korean army train at an undisclosed location in March 2013.

Kim arrives at Jangjae Islet by boat to meet with soldiers of the Jangjae Islet Defense Detachment in March 2013.

Kim arrives at Jangjae Islet by boat to meet with soldiers of the Jangjae Islet Defense Detachment in March 2013.

Kim is surrounded by soldiers during a visit to the Mu Islet Hero Defense Detachment, also near Taeyonphyong Island, in March 2013.

Kim is surrounded by soldiers during a visit to the Mu Islet Hero Defense Detachment, also near Taeyonphyong Island, in March 2013.

Kim is greeted by a soldier's family as he inspects the Jangjae Islet Defense Detachment in March 2013.

Kim is greeted by a soldier's family as he inspects the Jangjae Islet Defense Detachment in March 2013.

Kim uses a pair of binoculars to look south from the Jangjae Islet Defense Detachment, near South Korea's Taeyonphyong Island, in March 2013.

Kim uses a pair of binoculars to look south from the Jangjae Islet Defense Detachment, near South Korea's Taeyonphyong Island, in March 2013.

Kim, with North Korean soldiers, makes his way to an observation post in March 2013.

Kim, with North Korean soldiers, makes his way to an observation post in March 2013.

In this KCNA photo, Kim inspects naval drills at an undisclosed location on North Korea's east coast in March 2013.

In this KCNA photo, Kim inspects naval drills at an undisclosed location on North Korea's east coast in March 2013.

Kim is briefed by his generals in this undated photo. On the wall is a map titled "Plan for the strategic forces to target mainland U.S."

Kim is briefed by his generals in this undated photo. On the wall is a map titled "Plan for the strategic forces to target mainland U.S."

A North Korean soldier, near Sinuiju, gestures to stop photographers from taking photos in April 2013.

A North Korean soldier, near Sinuiju, gestures to stop photographers from taking photos in April 2013.

Kim visits the Ministry of People's Security in 2013 as part of the country's May Day celebrations.

Kim visits the Ministry of People's Security in 2013 as part of the country's May Day celebrations.

Kim inspects a military factory in this undated picture released by the KCNA in May 2013.

Kim inspects a military factory in this undated picture released by the KCNA in May 2013.

Kim inspects the command of an army unit in this undated photo released Sunday, January 12, by the KCNA.

Kim inspects the command of an army unit in this undated photo released Sunday, January 12, by the KCNA.

A photo released by the KCNA on Thursday, January 23, shows the North Korean leader inspecting an army unit during a winter drill.

A photo released by the KCNA on Thursday, January 23, shows the North Korean leader inspecting an army unit during a winter drill.

A North Korean soldier kicks a pole along the banks of the Yalu River on Tuesday, February 4.

A North Korean soldier kicks a pole along the banks of the Yalu River on Tuesday, February 4.

A North Korean soldier uses binoculars on Thursday, February 6, to look at South Korea from the border village of Panmunjom, which has separated the two Koreas since the Korean War.

A North Korean soldier uses binoculars on Thursday, February 6, to look at South Korea from the border village of Panmunjom, which has separated the two Koreas since the Korean War.

A picture released Tuesday, March 18, by the KCNA shows Kim attending a shooting practice at a military academy in Pyongyang.

A picture released Tuesday, March 18, by the KCNA shows Kim attending a shooting practice at a military academy in Pyongyang.

In this photo released Thursday, April 24, by the Korean Central News Agency, Kim smiles with female soldiers after inspecting a rocket-launching drill at an undisclosed location.

In this photo released Thursday, April 24, by the Korean Central News Agency, Kim smiles with female soldiers after inspecting a rocket-launching drill at an undisclosed location.

A North Korean soldier patrols the bank of the Yalu River, which separates the North Korean town of Sinuiju from the Chinese border town of Dandong, on Saturday, April 26.

A North Korean soldier patrols the bank of the Yalu River, which separates the North Korean town of Sinuiju from the Chinese border town of Dandong, on Saturday, April 26.

Kim poses for a photo as he oversees a tactical rocket-firing drill in June.

Kim poses for a photo as he oversees a tactical rocket-firing drill in June.

Kim tours a front-line military unit in this image released Wednesday, July 16, by the KCNA.

Kim tours a front-line military unit in this image released Wednesday, July 16, by the KCNA.

A picture released by the KCNA shows Kim and his wife watching a performance by the Moranbong Band on Wednesday, September 3, in Pyongyang.

A picture released by the KCNA shows Kim and his wife watching a performance by the Moranbong Band on Wednesday, September 3, in Pyongyang.

This undated photo, released Tuesday, October 14, by the KCNA, shows Kim inspecting a housing complex in Pyongyang, North Korea. International speculation about Kim went into overdrive after he failed to attend events on Friday, October 10, the 65th anniversary of the Workers' Party. He hadn't been seen in public since he reportedly attended a concert with his wife on September 3.

This undated photo, released Tuesday, October 14, by the KCNA, shows Kim inspecting a housing complex in Pyongyang, North Korea. International speculation about Kim went into overdrive after he failed to attend events on Friday, October 10, the 65th anniversary of the Workers' Party. He hadn't been seen in public since he reportedly attended a concert with his wife on September 3.

Kim sits in the pilot's seat of a fighter jet during the inspection.

Kim sits in the pilot's seat of a fighter jet during the inspection.

Kim is seen walking with a cane in this image released Thursday, October 30, by the state-run Korean Central News Agency.

Kim is seen walking with a cane in this image released Thursday, October 30, by the state-run Korean Central News Agency.

A picture released by the North Korean Central News Agency shows North Korean leader Kim Jong Un appearing without his cane at an event with military commanders in Pyongyang on Tuesday, November 4. Kim, who recently disappeared from public view for about six weeks, had a cyst removed from his right ankle, a lawmaker told CNN.

Kim speaks during a meeting of the Political Bureau of the Central Committee of the Workers' Party of Korea in Pyongyang, North Korea, in this photo released February 19 by the state-run Korean Central News Agency.

Kim speaks during a meeting of the Political Bureau of the Central Committee of the Workers' Party of Korea in Pyongyang, North Korea, in this photo released February 19 by the state-run Korean Central News Agency.

Kim inspects a drill for seizing an island at an undisclosed location in North Korea in an undated picture released by North Korea's official Korean Central News Agency on February 21.

Kim inspects a drill for seizing an island at an undisclosed location in North Korea in an undated picture released by North Korea's official Korean Central News Agency on February 21.

Kim visits the Kumsusan Palace of the Sun in Pyongyang, North Korea, on April 15 to celebrate the 103rd birth anniversary of his grandfather, North Korean founder Kim Il Sung.

Kim visits the Kumsusan Palace of the Sun in Pyongyang, North Korea, on April 15 to celebrate the 103rd birth anniversary of his grandfather, North Korean founder Kim Il Sung.

Kim Jong Un, center, poses with soldiers on the snow-covered top of Mount Paektu in an April 18 photo released by South Korean news agency Yonhap.

Kim Jong Un, center, poses with soldiers on the snow-covered top of Mount Paektu in an April 18 photo released by South Korean news agency Yonhap.

Kim stands on the snow-covered top of Mount Paektu in North Korea in a photo taken by North Korean newspaper Rodong Sinmun on April 18 and released the next day by South Korean news agency Yonhap. Kim scaled the country's highest mountain, North Korean state-run media reported, arriving at the summit to tell soldiers that the hike provides mental energy more powerful than nuclear weapons.

Kim stands on the snow-covered top of Mount Paektu in North Korea in a photo taken by North Korean newspaper Rodong Sinmun on April 18 and released the next day by South Korean news agency Yonhap. Kim scaled the country's highest mountain, North Korean state-run media reported, arriving at the summit to tell soldiers that the hike provides mental energy more powerful than nuclear weapons.

North Korean leader Kim Jong Un meets with North Korea's first female fighter jet pilots in this undated photo released by the country's state media on Monday, June 22. He called the women "heroes of Korea" and "flowers of the sky."

North Korean leader Kim Jong Un meets with North Korea's first female fighter jet pilots in this undated photo released by the country's state media on Monday, June 22. He called the women "heroes of Korea" and "flowers of the sky."

Further details on the circumstances of the defection weren't immediately available. There was no reaction to the news on North Korean state media.

Border crossings are risky and rare

The border between North and South Korea is considered to be the most heavily militarized in the world. The two countries technically remain at war, because the 1950-53 Korean War ended in an armed truce and not a peace treaty.

North Korean soldiers have occasionally made it across the highly sensitive zone in the past.

In 2012, one managed to cross the border unnoticed and knock on the door of a South Korean barracks , embarrassing the South Korean military and prompting a public apology from the defense minister at the time for the security lapse.

But defections directly across the border from North to South Korea are rare.

Defection harder under Kim Jong Un

Most people attempt to flee the impoverished, oppressively ruled North by crossing the more open border into China and then trying to make their way to South Korea through other countries.

More than 26,000 refugees from the North have reached the South since the end of the Korean War, according to the Unification Ministry in Seoul.

The vast majority of them arrived during the past 15 years. But defection has become harder in recent years, the Unification Ministry says, and the number of people doing it has fallen significantly since Kim Jong Un succeeded his father as the North Korean leader.

North Korea says it's willing to talk

Separately, North Korea on Monday said it was willing to hold talks with the South -- but with conditions attached.

"If an atmosphere of trust and reconciliation is created between North and South Korea, there is no reason not to hold dialogue and negotiation between the two sides," a North Korean government statement carried by state media said.

The conditions it set out included its often repeated call for South Korea to stop holding joint military exercises with the United States, a request that Seoul and Washington have consistently dismissed.

2 South Koreans to be released

Pyongyang also told Seoul on Monday that it plans to release two South Korean citizens this week, according to the South Korean Unification Ministry.

North Korea said the two South Koreans -- a man in his late 50s with the surname Lee, and a woman in her early 50s with the surname Jin -- illegally entered North Korea on May 11. North Korea detained them the same month.

The South Korean government said it believes the two went missing while traveling in China near the North Korean border. It expects to collect them Wednesday morning at Panmunjom, the "truce village" that straddles the border between the two Koreas.