heightened, high
The country has put its troops on high alert.

full
amber, orange, red, yellow
His sudden disappearance triggered a red alert among his friends.

nationwide
bomb, fire, flood, pollution, security, terror
A security alert was issued after four men escaped from the prison.

email
Receive regular email alerts about breaking news.

issue, put out, raise, sound
They rang the bells to sound the alert.

spark (esp. BrE), trigger
call off
The alert was called off when it was found that the bomb was not live.

go out
A nationwide alert went out for three escaped prisoners.

on the alert
alert for
You should always be on the alert for anyone who looks suspicious.

be on full alert
The security forces are now on full alert.

place sb on alert, put sb on alert
Thousands of police were put on full alert at all main roads leading to the city.

keep sb on alert
a state of alert
The army was yesterday placed on a state of alert in case of more riots.

Alert is used with these nouns as the object:
authority, police, press

be, look, seem
become
remain, stay
keep sb
The machine should help to keep the pilot alert.

extremely, fairly, very, etc.
fully
immediately, instantly, suddenly
There was a noise outside and he was suddenly alert.

always, constantly
mentally
He was as mentally alert as a man half his age.

to
Climbers need to be alert to possible dangers.

Alert is used with these nouns:
expression, reader