North and South Korea have exchanged gunfire at their border, raising tensions a day after North Korean leader Kim Jong-un ended an almost three-week absence from public life.

Multiple gunshots were fired from North Korea towards a guard post in South Korea at 7:41am (local time), the South's Joint Chiefs of Staff said in a statement.

South Korea responded by firing two shots towards North Korea.

No injuries were reported.

After weeks of intense speculation about Mr Kim's health and whereabouts, the country's official media published photographs and a report on Saturday that the leader had attended the completion of a fertiliser plant, the first report of his appearance since April 11.

Mr Kim had not been seen in public since presiding over a meeting about coronavirus.

Speculation about his health intensified after he missed an April 15 event commemorating the birthday of his grandfather, state founder Kim Il-sung, something he had not done since inheriting power from his father Kim Jong-il in late 2011.

The Koreas are split along a 248-kilometre-long, 4-kilometre-wide border called the demilitarised zone (DMZ) that was originally created as a buffer.

Unlike its name, the DMZ is the world's most heavily fortified border.

An estimated 2 million mines are peppered inside and near the DMZ, which is also guarded by barbed-wire fences, tank traps and combat troops on both sides.

In late 2018, the two Koreas began destroying some of their front-line guard posts and removing mines from the DMZ as part of steps to reduce tensions.

However the efforts stalled amid a deadlock in nuclear negotiations between Mr Kim and US President Donald Trump meant to convince North Korea to give up its arsenal in exchange for lifting economic sanctions.

The last time there was gunfire along the border was in 2017, when North Korea sprayed bullets at a soldier fleeing to South Korea.

Wires