SEOUL (AFP) - North Korea twice fired shells near the flashpoint Yellow Sea border with South Korea Wednesday, prompting warning shots from the Souths marines in response, Seouls military said. The incidents fuelled already high tensions along the disputed sea border, which saw bloody naval skirmishes in recent years and a deadly shelling attack on South Koreas Yeonpyeong island last November. Meanwhile, North Korea has sent secret agents to South Korea to try to assassinate its defence minister, who has vowed a stern response to attacks from the communist state, media reports said Wednesday.