South Korean news media say North Korea has test fired air-to-ship missiles in the Yellow Sea, prompting Seoul to step up its anti-air defenses.

The Yonhap news agency and local newspapers quote government sources as saying the North Korean military fired air-to-ship missiles in October and again this month using IL-28 bombers.

The reports say the missiles were a modified version of the Styx surface-to-ship missile, which has a range of 40 kilometers. The missiles are deployed along the coastline north of the inter-Korean sea border.

Yonhap quotes an official as saying Seoul is bolstering its anti-air defenses against any possible provocation by the North.

Tensions between the two Koreas rose last year when Seoul accused Pyongyang of sinking a warship near the disputed sea border, known as the Northern Limit Line. Forty-six South Korean sailors were killed in the March 2010 attack.

Relations worsened after the North bombarded South Korea's Yeopyeong island in November 2010, killing four people.

Tensions have eased in recent months. Seoul replaced a hardline minister in charge of cross-border relations in August and has delivered medical aid to the North.