Is sexual intimacy with one's wife after a Khul' divorce, and before taking her back, considered a رجعة (resumption of marriage) or zina (fornication)?

A khul‘ is an irrevocable divorce. There is no returning to her except with the permission of her guardian, a new marriage contract, a new dowry, and two witnesses. Your sexual intercourse with the woman after the khul‘ is a grave sin for which you must repent. You must not disclose what happened to anyone.