Tensions between North and South Korea have risen after they exchanged warning shots off the west coast of the peninsula on Monday.
Governments from both nations accused each other of breaching their maritime borders, in a sign the South may be taking a stricter line towards its rogue neighbour to the north.
A statement from the South's Joint Chiefs of Staff (JCS) said warnings had been broadcast and warning shots fired after a merchant vessel crossed the Northern Limit Line (NLL) at about 3:40am local time (6:40pm UK time).
Kim Jong-un's North Korea, meanwhile, said via state media that it had fired 10 rocket artillery rounds after a South Korean navy ship had violated the NLL and fired warning shots "on the pretext of tracking down an unidentified ship".
The official KCNA news agency reported a spokesperson for the General Staff of the North's Korean People's Army as saying: "We ordered initial countermeasures to strongly expel the enemy warship."
But the South's JCS said it had conducted a "normal operation", describing the North's move as a violation of a bilateral military pact in 2018 which had banned "hostile acts" in the border areas.
"We once again urge North Korea to immediately cease consistent provocations and accusations which harm the peace and stability of the Korean peninsula as well as the international community," the JCS said in a statement.
The latest flashpoint comes as Kim Jong-un's nation carries out weapons tests at an unprecedented pace.
In recent days, short-range ballistic missiles and hundreds of artillery rounds have been fired off its east and west coasts to protest the South's annual Hoguk defence drills, which are scheduled to run until Wednesday.
Earlier this month, residents in Japan were told to take shelter after missiles landed in the Pacific Ocean.
Officials in Tokyo and Seoul said the missile flew between 4,500 to 4,600 kilometres to a maximum altitude of about 1,000 km.
South Korea's President Yoon Suk-yeol warned of a "resolute" response.
