Is it permissible to say "I love you" and other affectionate phrases to one's niece, considering her as a daughter, out of compassion, and to protect her from being influenced by the words of non-mahrams?

The paternal uncle is a mahram to his brother's daughter by consensus of scholars. It is permissible for him to address her with expressions that indicate his affection for her, as long as there is no fear of fitna. A group of scholars also held the view that it is permissible for a man to kiss his mahram if fitna is avoided, but not on the mouth; rather, on the forehead or the head, as the Prophet, peace and blessings be upon him, did with Fatima upon his return from battle.