JERUSALEM/CAIRO, Mar. 18 (Reuters) - Israeli airstrikes pounded Gaza and killed more than 400 people on Tuesday, Palestinian health authorities said, shattering nearly two months of relative calm since a ceasefire began, as Israel warned the onslaught was "just the beginning."

Israel and Palestinian militant group Hamas accused each other of breaching the truce. It had broadly held since January and offered respite from war for the 2.3 million inhabitants of Gaza, which has been reduced to rubble.

Hamas, which still holds 59 of the 250 or so hostages Israel says the group seized in its October 7, 2023 attack, accused Israel of jeopardising efforts by mediators to negotiate a permanent deal to end the fighting, but the group made no threat of retaliation.

Israeli Prime Minister Benjamin Netanyahu said he ordered strikes because Hamas had rejected proposals to secure a ceasefire extension.

Netanyahu called on Gazans to get out of harm's way and move to safer areas, blaming every civilian casualty on Hamas.

"From now on, Israel will act against Hamas with increasing force. And from now on, negotiations will only take place under fire," he said, speaking from the Kirya army base in Tel Aviv.

"Hamas has already felt the blow of our arm in the last 24 hours. And I want to assure you: This is just the beginning."

The strikes hit houses and tent encampments from the north to the south of the Gaza Strip. Witnesses said an Israeli plane fired missiles into Gaza City late on Tuesday.

Israeli tanks shelled from across the border, witnesses said. Palestinian health authorities said 408 people had been killed in one of the biggest single-day tolls since the war erupted.

"It was a night of hell. It felt like the first days of the war," said Rabiha Jamal, 65, a mother of five from Gaza City.

Photo from Reuters