The military said Sunday that four small boats attacked a cargo ship and fired at responding U.S. helicopters, which returned fire and sank three of the boats.

Israel’s unprecedented air and ground offensive has killed more than 21,800 Palestinians and wounded more than 55,000 others, according to the Health Ministry in Hamas-ruled Gaza.

Israel’s widening campaign, which has already flattened much of the north, is now focused on the urban refugee camps of Bureij, Nuseirat and Maghazi in central Gaza

But after weeks of Israel’s blistering air and ground offensive, only 9 out of Gaza’s 36 hospitals are still operational.

Despite U.S. calls for Israel to curb civilian casualties and international pressure for a cease-fire, Israeli Prime Minister Benjamin Netanyahu said the military was deepening the fighting.