Standardised European Rules of the Air (SERA):  Action by all other ATS units/aircraft 

(i) The distress communications have absolute priority over all other communications and ATS units/aircraft aware of them shall not transmit on the frequency concerned unless: 

(A) the distress is cancelled or the distress traffic is terminated; 

(B) all distress traffic has been transferred to other frequencies; 

(C) the ATS unit controlling communications gives permission; 

(D) it has itself to render assistance.

ICAO Annex 10, Volume II 
5.3.2.2 Action by the station addressed or first 5.3.2.2 station acknowledging the distress message

5.3.2.2.1 The station addressed by aircraft in distress, or first station acknowledging the distress message, shall:

a) immediately acknowledge the distress message; 
b) take control of the communications or specifically and clearly transfer that responsibility, advising the aircraft if a transfer is made; 
c) take immediate action to ensure that all necessary information is made available, as soon as possible, to:

1) the ATS unit concerned; 
2) the aircraft operating agency concerned, or its representative, in accordance with pre-established arrangements;

Note.— The requirement to inform the aircraft operating agency concerned does not have priority over any other action which involves the safety of the flight in distress, or of any other flight in the area, or which might affect the progress of expected flights in the area.

d) warn other stations, as appropriate, in order to prevent the transfer of traffic to the frequency of the distress communication.