Gaza’s fragile ceasefire was shattered early Tuesday as Israel carried out deadly strikes across the enclave and Israeli Prime Minister Benjamin Netanyahu vowed to use “increasing military strength” against Hamas.

DEIR AL-BALAH, Gaza Strip (AP) — Israel launched airstrikes across the Gaza Strip early Tuesday, killing at least 400 Palestinians, mostly women and children, according to local health officials. The surprise bombardment, the deadliest in Gaza since the start of the 17-month war, shattered a ceasefire in place since January.

Prime Minister Benjamin Netanyahu’s office said he had ordered the aerial attacks after Hamas’s “repeated refusal” to release the remaining hostages it holds. “This is just the beginning,” he said.