North Korea fired artillery into the sea near its disputed border with South Korea for the third day today. Seoul's Joint Chiefs of Staff told AFP that the communist state fired 20 shells.<br/> <br/>He said shells landed in North Korean waters off South Korea's Yeonpyeong island. Seoul's Defence Ministry said it is considering shipping more artillery and advanced radar to two border islands in response to the shelling, which began on Wednesday near the tense frontier in the Yellow Sea.<br/><br/>The North said it is conducting a routine artillery drill, but Seoul and Washington criticise the gunfire as provocative.