Israel launched air strikes in Gaza late on Tuesday, saying it mounted the strikes after an attack by Palestinian militants killed one soldier, in the latest challenge to an already fragile ceasefire.
The Israeli military said it would continue to uphold the ceasefire agreement and would respond firmly to "any violation".
US President Donald Trump said that the US-backed ceasefire was not at risk even as Israeli planes struck across Gaza, with Israel and militant group Hamas blaming each other for breaches of the truce.
Gazan health authorities said the Israeli strikes killed at least 104 people, including five in a house hit in the Bureij refugee camp in the central Gaza Strip, four in a building in Gaza City's Sabra neighbourhood, and five in a car in Khan Younis.
"As I understand it, they took out an Israeli soldier," Trump told reporters aboard Air Force One.
"So the Israelis hit back and they should hit back. When that happens, they should hit back."
The Israeli military confirmed the soldier's death on Wednesday.
"Nothing is going to jeopardise" the ceasefire, Trump said, adding Hamas "have to behave".
An Israeli military official said Hamas had violated the ceasefire by carrying out an attack against Israeli forces who were stationed within the so-called "yellow line", the deployment line agreed upon in the ceasefire.
The agreement went into effect on October 10, halting two years of war triggered by deadly Hamas-led attacks on Israel on October 7, 2023.
"If they (Hamas) are good, they are going to be happy and if they are not good, they are going to be terminated, their lives will be terminated," Trump said.
"Nobody knows what happened to the Israeli soldier but they say it was sniper fire. And it was retribution for that, and I think they have a right to do that."
Hamas denied responsibility for the attack on Israeli forces in Rafah, in southern Gaza, and said it remained committed to the ceasefire deal.
Under the accord, Hamas released all living hostages in return for nearly 2000 Palestinian prisoners and wartime detainees, while Israel pulled back its troops and halted its offensive.
Hamas also agreed to hand over the remains of all dead hostages yet to be recovered, but has said it will take time to find and retrieve all of the bodies.
Israel says the militant group can access the remains of most of the hostages.
The issue has become one of the main sticking points in the ceasefire.
Israeli Prime Minister Benjamin Netanyahu said human remains handed over on Monday night belonged to an Israeli killed during Hamas's October 7 attack, whose body was recovered by Israeli forces in the early weeks of fighting.
The Israeli military said that Hamas had planted the remains at an excavation site before calling in a Red Cross team and pretending it had found a missing hostage, to create a "false impression of efforts to locate bodies".
A 14-minute video published by the military showed three men placing a white bag at an excavation site and then covering it with earth and rocks.
Reuters was able to independently confirm the location of the video, but was unable to verify the date of the video or Israel's account of what the video showed.
Hamas did not immediately respond to a request for comment.
The ICRC said its team was unaware that the remains had been planted at the site before their arrival.
