Asking for user input until it's valid without going back to main loop

I have the following piece of code asking for a certain input in a larger program:

elif user_input == "s":
    input_sequence = input("Please enter a three letter sequence: ")
    if len(input_sequence) == 3  and user_input.isalpha():
        letter1 =  input_sequence[0]
        letter2 =  input_sequence[1]
        letter3 =  input_sequence[2]
        segment_sequence(corpus, letter1, letter2, letter3)
    else:
        print("\nERROR: please enter a three letter sequence\n")

However when I use the conditional statements, if an invalid input is entered I return back to my main while loop. How would I be able to go from an invalid input back to this same piece of code?

Apologies for any potential confusion in terminology as I'm new to programming.

Many thanks