International Criminal Court prosecutor Karim Khan is "deeply concerned by the reported bombardment and potential ground incursion by Israeli forces in Rafah", he said on social media platform X on Monday.
He reiterated that the ICC was "actively investigating any crimes allegedly committed" in Gaza and said that "those who are in breach of the law will be held accountable".
Prime Minister Benjamin Netanyahu's office has said four Hamas battalions are in Rafah, where hundreds of thousands of people displaced by violence further north are trapped in desperate conditions.
It says Israel cannot achieve its goal of eliminating the militants while they remain there. Civilians should be evacuated from the combat zone, it said.
Khan also called for the immediate release of all the hostages held in Gaza after Hamas' deadly Oct. 7 attacks on Israel, adding that this "also represents an important focus of our investigations".
Israel is not a member of the Hague-based court and does not recognize its jurisdiction. But Khan in October stressed his court had jurisdiction over any potential war crimes carried out by Hamas in Israel and by Israelis in the Gaza Strip.