SEOUL (Reuters) - A North Korean soldier defected to South Korea on Thursday, the South’s military said, making a rare crossing of one of the world’s most heavily fortified borders amid heightened tension between the rival neighbors.

The soldier crossed the military demarcation line that runs through the demilitarized zone (DMZ) dividing the two Koreas, which remain in a technical state of war since their 1950-53 conflict ended in a truce, not a peace treaty.

While on average more than 1,000 North Koreans defect to South Korea every year, most travel via China and it is unusual for a North Korean to cross the DMZ, which is heavily mined and lined with barbed wire and soldiers on both sides.

The last such crossing was in June 2015.

The soldier was unarmed and there was no exchange of fire, a South Korean military official said.

He walked across an eastern part of the front line at around 10 a.m. (0100 GMT) and was being questioned over how and why he made the crossing, South Korea’s Office of Joint Chiefs of Staff said in a statement.

The escape follows a series of high profile defections that have angered North Korea at a time of heightened international tension over its nuclear and missile tests.

The most prominent defector was Thae Yong Ho, the North’s deputy ambassador to Britain, who became the highest-ranking diplomat to defect to the South when he arrived last month, an embarrassing blow to the government of North Korean leader Kim Jong Un.

In April, 12 North Korean waitresses in a restaurant in China fled to the south, along with their manager.

Tension has been high on the Korean peninsula since the North conducted its fourth nuclear test at the beginning of the year and followed it with an unprecedented string of missile tests, and then its fifth and largest nuclear test this month.

That drew widespread condemnation, prompting global powers to seek toughened sanctions against the isolated country.

Separately, media reported that a North Korean teenager who took part in a Hong Kong maths contest in July and later sought asylum at the South Korean consulate there had arrived in South Korea.

South Korea declined to confirm or comment on the report.