Militants in the Gaza Strip have fired more than 90 rockets into Israel, the army says, prompting air strikes and tank fire on the Palestinian territory.
Sirens went off as people rushed to shelters in southern Israel. There are no reports of casualties there, but one Palestinian has been been killed.
Four Palestinians, including two Hamas militants, were killed on Friday after an attack injured two Israeli soldiers.
The flare-up followed a lull in the run-up to Israeli elections in April.
It marks yet another increase in hostilities despite attempts by Egypt and the United Nations to broker a longer-term truce, says the BBC’s Tom Bateman in Jerusalem.
Hamas has sought to ease the Israeli blockade of Gaza and accused Israel of failing to implement last month’s truce deal.
Israel says the tight blockade is needed to stop weapons reaching Gaza.
Saturday’s rocket barrage sent Israelis scrambling for safety and Israeli media have shown damage caused to houses in Ashkelon.
The country’s Iron Dome missile-defence system shot down dozens of the rockets, the Israel Defence Forces (IDF) said.
The IDF said its war planes targeted at least five sites in Gaza belonging to Hamas and Islamic Jihad militants. Tanks were also targeting militants, it said.
Palestinian officials say a 22-year-old man has been killed and several injured.
The upsurge in violence began during weekly Friday protests by Palestinians opposed to the Israeli blockade.
A Palestinian gunman shot and wounded two Israeli soldiers at the boundary fence.
The Israeli air strike in response killed two Hamas militants. Another two Palestinians were killed by Israeli fire at the fence.
Saturday’s rockets attacks coincided with Palestinians burying the two militants.
“The resistance will continue to respond to the crimes by the occupation and it will not allow it to shed the blood of our people,” Hamas spokesman Abdel-Latif al-Qanoua said in a statement on Saturday. He made no explicit claim for Hamas firing the rockets.
Around two million Palestinians live in Gaza, which has suffered economically from the Israeli blockade as well as recent foreign aid cuts.