The crew members told authorities they had crossed the maritime border due to a "navigation error and mechanical problem" and asked to return to the North, the defence ministry said.

"Our military handed over all seven North Korean crew and the ship to the North... on humanitarian grounds and according to the custom," it added in a statement.

A North Korean patrol boat had also crossed the de facto maritime border off the west coast of the peninsula early Tuesday while chasing the southbound vessel, a defence ministry official said.

The South's navy fired a warning shot at the North Korean patrol boat, which turned around and headed back to the North, the official added.

The incident comes as South Koreans vote Wednesday in a presidential election, as tensions mount with the North, which has embarked on a record-breaking blitz of weapons tests this year.

Leif-Eric Easley, a professor at Ewha University in Seoul warned that "repatriation won't be simple" for the returning North Korean crew, due to the pandemic.

"North Korea has strictly isolated its poor health care infrastructure and unvaccinated population from the global pandemic. Individuals who have been in South Korea as that country reports new record Covid numbers may not be welcome," he said.

South Korea is in the grip of an Omicron wave, with more than 200,000 new cases recorded most days this month.

More than a million people are currently isolating at home after testing positive, health authorities say.

The West Sea off the Korean peninsula has been the site of military clashes between the two Koreas along the so-called Northern Limit Line border, which Pyongyang does not recognise.

One of the most deadly incidents involved a North Korean torpedo attack on a South Korean warship in 2010, which killed 46 sailors. Pyongyang denies responsibility for the attack.