GENEVA - South Korea called on Tuesday for "collective measures" against North Korea, including possible suspension of its UN membership, saying the use of chemical weapons to assassinate the half-brother of North Korea's leader was a "wake-up call."Yun Byung-se, South Korea's foreign minister, told the UN-sponsored Conference on Disarmament: "North Korea is reported to have not just grams but thousands of tonnes of chemical weapons including VX all over the country ... The recent assassination is a wake-up call to all of us to North Korea's chemical weapons capability and its intent to actually use them."Yun said that states could invoke the Chemical Weapons Convention and take "collective measures." "It could take the form of suspension of North Korea's rights and privileges as a UN member," he said, calling South Korea's isolated neighbor a "serial rule-breaker."