A North Korean soldier defected to South Korea on Thursday through the heavily guarded demilitarized zone separating the two countries, leading to gunfire on both sides of the border, the South Korean military said.

The "low ranking” soldier was manning a guard post along the DMZ when he fled through thick fog, the South Korean military’s Joint Chiefs of Staff said.

The escape follows a similar one last month, in which another North Korean soldier was shot by his colleagues as he successfully fled his DMZ posting.

In that case, South Korean border guards who heard the gunshots found the soldier 55 yards from the border line that bisects Panmunjom, the so-called truce village in the Joint Security Area, and carried him to safety. Doctors later found dozens of parasitic worms in his digestive system, some as long as 11 inches, which officials said was a sign of the poor hygiene and nutrition afflicting North Korea.

Officials said the soldier who fled Thursday was not fired upon. South Korean soldiers later fired 20 warning shots at North Korean border guards who were searching for the defector, which was followed 40 minutes later by gunfire in the North, the Joint Chiefs of Staff said.

More than 30,000 North Koreans have fled to South Korea since a famine killed at least a million people in the North in the 1990s. The country has recently been the subject of a tightening grip of sanctions that have curbed exports that provide urgently needed revenue, and it has been struggling with the impact of a drought that has reduced agricultural yields.

But it is extremely rare for people to flee across the demilitarized zone. The 2.5-mile-wide DMZ, considered the most heavily fortified border in the world, is guarded by minefields, sentry posts and tall fences topped with barbed wire, some electrified.

The soldier’s defection to South Korea on Thursday was the fourth this year, and the gunfire over the episode is certain to raise tensions on the Korean Peninsula just as hopes have grown for a thaw in relations between the South and North.

South Korean officials have recently held out the possibility that they might be willing to push back the timing of planned joint military exercises with the United States to reduce tensions.

Those exercises have traditionally drawn a fierce response from North Korea, which sees them as a preparation for military action against the North.

Agencies