South Korea stayed on guard against North Korea on Tuesday, with jet fighters and warships deployed, a day after the communist state backed off from threats of attack over a live-fire exercise. Hours after the South defied North Korea's threats and staged artillery exercise on Monday near the disputed sea border, the North announced it did not feel any need to retaliate against every despicable military provocation. The comments eased fears of war on the peninsula, following almost a month of high tensions.South Korea's Defence Minister Kim Kwan-Jin told parliament on Tuesday that this was the most serious crisis in Seoul's national defence since the 1950-53 Korean War. Kim, who has promised strong retaliation using air power for any future strike, said a thorough military readiness was maintained at sea including Yeonpyeong Island against possible provocations by the North. He said jet fighters were still on patrol while his ministry said warships are on standby in the Yellow Sea.