North Korea recently test fired new air-to-ship missiles in the Yellow Sea, prompting South Korea to step up anti-air defences, a news report said Wednesday.

"The North Korean military test fired two air-to-ship missiles from IL-28 bombers, one in October and the other early this month," a government source was quoted as telling Yonhap news agency.

A defence ministry spokesman declined to comment on the report.

The missiles were a modified version of Styx ground-to-ship missiles which the North deploys along its west coast, with a 40-kilometre (25 mile) range, the report said.

"If the North fires air-to-ship missiles from IL-28 bombers near the NLL (Northern Limit Line), it would pose a grave threat to our patrol boats and destroyers manoeuvring south of the NLL," the source said.

"Military authorities are bolstering anti-air defence on the ground and on ships," the source added without giving details.

North Korea does not recognize the NLL as a legitimate sea border as it was unilaterally drawn by the US-led United Nations Command after the Korean War. Deadly naval clashes erupted near the line in 1999, 2002 and 2009.

Tensions rose when the South accused the North of sinking a warship near the disputed border and killing 46 sailors in March 2010.

They worsened further after the North bombarded Yeonpyeong island just south of the borderline and killed four South Koreans in November 2010.

Following the attack, South Korea reportedly deployed its ground-to-air Cheonma missiles on Yeonpyeong and Baengnyeong islands.

In recent months there have been signs of an easing of tensions. Seoul replaced a hardline minister in charge of cross-border relations and has delivered medical aid to its neighbour.