An approved flight crew compartment door shall be capable of being locked and unlocked from either pilot's station and designed to meet the applicable airworthiness requirements. Means should be provided for monitoring from either pilot’s station the entire door area outside the flight crew compartment to identify persons requesting entry and to detect suspicious behaviour or potential threat.

In all aeroplanes which are equipped with a flight crew compartment door, this door shall be capable of being locked, and means shall be provided by which cabin crew can discreetly notify the flight crew in the event of suspicious activity or security breaches in the cabin.

Source: ICAO Annex 6 - 13.2 Security of the flight crew compartment.