?warn;admonish ?be on the alert against;guard against;keep a close watch on 采取～措施take precautionary measures /～部隊(duì)outpost troops;vigilance corps;security force (or detachment)/～級(jí)別warning level/～哨outguard; picket guard/～水位warning line; warning water level/～線danger mark;security line;cordon/～狀態(tài)state of alert