SEOUL: South Korea said on Wednesday (Aug 25) it had scrambled fighter jets in response to Russian military aircraft entering its air defence zone, the second such incursion in weeks.

The relationship between the countries has worsened since South Korea joined Western sanctions following Russia's invasion of Ukraine, and since Moscow built stronger ties with North Korea.

Seoul's defence ministry said multiple Russian aircraft had entered and then left its air defence identification zone over the East Sea, also called the Sea of Japan.

"South Korea's military detected the Russian aircraft before they entered the zone and deployed Air Force fighter jets to prepare for any contingency," it said, without giving details.

The Russian aircraft did not violate South Korean airspace, it said.

An air defence identification zone is not sovereign airspace but a buffer area where countries require foreign aircraft to identify themselves for security purposes.

Military aircraft are generally expected to notify the relevant country before entering its zone, although there is no legal obligation to do so.

In late June, more than 10 Chinese and Russian military aircraft entered South Korea's air defence zone.

Beijing's defence ministry said at the time that Chinese and Russian forces had conducted a "strategic air patrol" over the Sea of Japan, the East China Sea and the western Pacific Ocean.

Russia's defence ministry said the patrol was part of normal military cooperation plans and "not directed against third countries".

But South Korea still lodged protests with Beijing and Moscow and demanded measures to prevent a recurrence.