Buckingham Palace said the King was "extremely concerned" and his thoughts and prayers were with all who were suffering.
Israel has vowed unprecedented retaliation against the Palestinian militant group Hamas after its terrorists stormed through the Gaza border on Saturday.
They went on to kill hundreds, including babies, in their homes, on streets and at a music festival.
The Jewish state has launched a barrage of air strikes on the territory and stopped food, water, fuel and medicine supplies.
The violence has already claimed at least 2,100 lives on both sides, with fears it could escalate further.
A Palace spokesman said: "This is a situation His Majesty is extremely concerned about and he has asked to be kept actively updated. His thoughts and prayers are with all of those suffering, particularly those who have lost loved ones, but also those actively involved as we speak."
He added: "His Majesty is appalled by and condemns the barbaric acts of terrorism in Israel."
Distressed
The King, 74, spoke to Israel's president, Isaac Herzog, and King Abdullah of Jordan yesterday, to express condolences and try to encourage key Middle East players to avoid a wider confrontation.
Sources close to the King stressed his lifelong efforts to encourage inter-faith and community dialogue, peace and understanding, both nationally and internationally.
He is supported by Prince William, 41, who visited the Holy Land in 2018, and wife Kate.
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