Among the dead was a five-year-old and 44 people had been wounded in the strikes, added officials. Palestinian officials claimed one of those who had died was a senior militant.
Israel said it was targeting the Islamic Jihad group and the strikes came after tensions were heightened following the arrest of a senior militant, Bassam al-Saadi, in the occupied West Bank earlier this week.
“The Israeli government will not allow terrorist organisations in the Gaza Strip to set the agenda in the area adjacent to the Gaza Strip and threaten the citizens of the State of Israel,” Prime Minister Yair Lapid said in a statement.
“Anyone who tries to harm Israel should know: We will find you.”
Israel said it was targeting militants in an operation named “Breaking Dawn”. It also announced a “special situation” on the home front, with schools closed and limits placed on other activities in communities within 50 miles of the border.
The strikes risk igniting yet another war in the territory, which is ruled by the Islamic militant group Hamas and is home to about two million Palestinians. The assassination of a senior militant would likely be met by rocket fire from Gaza, pushing the region closer to all-out war.
Islamic Jihad said in a statement that it would seek revenge for the attack.
“The enemy has begun a war against our people and against us and we will defend ourselves and our people,” it said.
Officials said Tayseer al-Jaabari, a senior commander in the movement, had been killed.
A spokesman for the Gaza Health Ministry confirmed there had been a number of casualties and a state of emergency had been declared. Television pictures showed black smoke rising from the roof of a building in the heavily populated strip.
Israel and the Hamas fought four wars and several smaller skirmishes in the 15 years since the militant group seized power in the coastal strip from rival Palestinian forces.
The most recent was in May 2021.
Islamic Jihad leader Ziad al-Nakhalah, speaking to the al-Mayadeen TV network from Iran, said: “We are starting the fight, and the fighters of the Palestinian resistance have to stand together to confront this aggression”.
He said there would be “no red lines” in the confrontation and blamed the violence on Israel.
Hamas spokesman Fawzi Barhoum said “the Israeli enemy, who started the escalation against Gaza and committed a new crime, must pay the price and bear full responsibility for it”.
Islamic Jihad is opposed to Israel’s existence and has carried out attacks over the years, including the firing of rockets into southern Israel. It is unclear how much control Hamas has over Islamic Jihad, and Israel holds Hamas responsible for all attacks emanating from Gaza.