high alert; severe warning
Used in official or news contexts to describe a state of heightened vigilance or a strong cautionary notice.
With the typhoon approaching, the meteorological agency is calling for high alert.
The police placed themselves on high alert against the possibility of terrorism.
警戒 is a general term for 'vigilance' or 'caution'; 厳重警戒 adds the nuance of 'strict' or 'severe', making it a stronger, more formal warning.
厳重注意 is a 'stern warning' or 'severe admonition' directed at a person or group, while 厳重警戒 is a state of alertness against a threat or danger.
Compound of 厳重 (strict, severe) and 警戒 (vigilance, caution). The exact historical derivation is uncertain, but the word follows standard Sino-Japanese compounding patterns.