The North Korean patrol boat crossed the de facto maritime border off the west coast of the peninsula early Tuesday while chasing a vessel that was heading south, a defence ministry official said.

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

The seven crew of the seized boat are being questioned by authorities, the defence ministry official said.

Yonhap reported that the crew members told authorities that they had crossed the maritime border due to a "navigation error" and "strongly demanded a return" to the North.

The defence official refused to confirm those details, citing an ongoing probe.

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

A patrol boat crossing the de-facto maritime border "will inevitably take on a political dimension because it occurred on the eve of South Korea's presidential election," said Leif-Eric Easley, a professor at Ewha University in Seoul.

North Korea typically demands immediate repatriation of citizens but this could be complicated by the pandemic, he said.

"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 being recorded on most days this month.

More than a million people are currently isolating at home after testing positive, health authorities say. The country amended its electoral laws last month to ensure they would be able to vote.

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.