U.S. President Joe Biden arrived in Israel on Wednesday, pledging solidarity in its war against Hamas and backing its account that a blast that killed huge numbers of Palestinians at a Gaza hospital had been caused by militants.

The fireball that engulfed the Al-Ahli al-Arabi hospital delivered some of the most harrowing images yet from a 12-day war and wrecked White House plans for Biden’s emergency diplomatic mission to the Middle East, with Arab leaders calling off their planned summit with the U.S. president.

Palestinian officials blamed an Israeli air strike for the blast, which they said killed as many as 500 people. Israel said the blast was caused by a failed rocket launch by the Palestinian Islamic Jihad militant group, which denied blame.

Speaking alongside Israeli Prime Minister Benjamin Netanyahu, Biden said: “I was deeply saddened and outraged by the explosion of the hospital in Gaza yesterday, and based on what I’ve seen, it appears as though it was done by the other team, not you.”