But since China signed up to the U.N.'s toughest sanctions against North Korea yet in March, after Pyongyang's fourth nuclear test, the daily caravan of trucks rumbling over Dandong's iron bridge from the North has slowed to a trickle.
My question is:How to explain "yet" in the paragraph cited from Time above? Does it mean "again"? But I can't find any record shows that China had done this to North Korea before. Thank you!