South Korea's army says it has conducted its largest-ever artillery drills near the tense border with North Korea.

The army says 300 artillery systems deployed by dozens of military units along the border began firing shells at the same time on Thursday afternoon.

It says North Korea's military hasn't shown any signs of unusual activities in response.

The army says the drills marked what South Korea describes as an artillery exchange between the Koreas in August 2015 following land mine explosions that maimed two South Korean soldiers.

There were no reports of injuries in last year's artillery firing. Seoul said North Korea fired first, but Pyongyang denied firing.

Thursday's drills come a day after Seoul said a senior North Korean diplomat based in London defected to South Korea.