Related Rule
South Africa
Practice Relating to Rule 20. Advance Warning
South Africa’s LOAC Manual (1996) states: “In terms of Article 57 [of the 1977 Additional Protocol I] there is a general requirement to provide a warning before an attack if civilians are present. An exception to the rule is if surprise is a key element of attack.” 
South Africa, Presentation on the South African Approach to International Humanitarian Law, Appendix A, Chapter 4: International Humanitarian Law (The Law of Armed Conflict), National Defence Force, 1996, § 28(g). This manual is also included in Chapter 4 of the Draft Civic Education Manual of 1997.
South Africa’s Revised Civic Education Manual (2004) states:
Warning requirements. In terms of Article 57 of Protocol I of the Geneva Conventions, there is a general requirement to provide a warning before an attack if civilians are present. An exception to this rule is if surprise is a key element of attack. 
South Africa, Revised Civic Education Manual, South African National Defence Force, 2004, Chapter 4, § 50(g).