Three members of the Hamas movement and a Palestinian were killed by Israel in the Gaza Strip on Friday.
Israeli aircraft and tanks hit targets across the coastal enclave in response to shots fired at troops on the border, the army said. Hamas announced that three of its members were killed.
A fourth Palestinian was shot dead during protests along the frontier with Israel, Gaza's health ministry said.
The flare-up is the latest in months of tensions that have ratcheted up fears of a wider conflict between Israel and Gaza's rulers Hamas, who have fought three wars since 2008.
An Israeli army statement said shots were fired at troops during renewed protests along the Gaza-Israel frontier and "in response... aircraft and tanks targeted military targets throughout the Gaza Strip."
It did not say if any Israeli soldiers were hurt in the shooting.
Two Palestinians were killed in a strike east of Khan Yunis in southern Gaza, the health ministry in the enclave said. A third man was killed by Israeli fire east of Rafah, also in southern Gaza.
The military wing of Hamas named the three men as Shaban abu Khatar, Mohammed abu Farhana and Mahmoud Qushta, saying they were fighters.
Israel's military said its aircraft and tanks had targeted "eight military posts" belonging to Hamas.
A fourth man named as Mohammed Badwan was later shot dead by Israeli forces during protests along the border, Gaza's health ministry said.
Israeli media said Prime Minister Benjamin Netanyahu was holding discussions with cabinet colleagues and military chiefs about the escalation of hostilities during the long-running border protests on the Israel-Gaza frontier.
Last weekend saw the most severe exchange of fire between Israel and Palestinian militants in Gaza since a 2014 war, raising worries of another round of conflict.
Israel hit dozens of sites it said belonged to militants in the Gaza Strip in Saturday's strikes, killing two Palestinian teenagers.
The same day, around 200 rockets and mortars were fired at Israel from Gaza and four Israelis were wounded when a rocket hit a house in the nearby Israeli city of Sderot.
Since protests broke out on March 30, at least 149 Palestinians have been killed.
Most were shot during demonstrations and clashes along the border, but others were killed in air strikes or by tank fire.
No Israelis have been killed.
