At least eight people have died, including a senior member of the Palestinian Islamic Jihad (PIJ), as a result of Israeli airstrikes, officials have said.
A blast was heard in Gaza City on Friday and smoke was seen coming from the seventh floor of a tall building.
The Palestinian Health Ministry said that 40 people were wounded, and a senior Islamic Jihad commander and a five-year-old child are among the dead.
The strike comes after days of tensions following the arrest of a senior militant in the occupied West Bank on Monday.
Israel had closed roads around Gaza earlier this week and sent reinforcements to the border, in anticipation of retaliation.
“The IDF (Israeli Defence Forces) is currently striking in the Gaza Strip. A special situation has been declared on the Israeli home front,” the Israeli military said in a statement.
The Palestinian Islamic Jihad (PIJ) and IDF said the official killed was Tayseer al-Jaabari.
Israel airstrikes target Gaza Strip in response to rocket attack
Israel carries out strikes on Gaza in retaliation for rocket fired from the strip
Israel launches strikes on Gaza in retaliation for rockets fired the previous day
According to the IDF, he was “responsible for multiple terrorist attacks against Israeli civilians” and shared a picture of him on social media.
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”.
In a statement broadcast on Al Mayadeen television, a pro-Iranian Lebanese channel, Islamic Jihad leader Ziad Al Nakhala vowed retaliation for the strikes.
“There are no red lines in this battle and Tel Aviv will fall under the rockets of resistance, as well as all Israeli cities,” he said.
Middle East correspondent
Tensions have been high around Gaza for days, following the arrest by Israel of the Palestinian Islamic Jihad (PIJ) leader in the West Bank earlier in the week.
An 80km zone of road closures and lockdown has been in place on the Israeli side, in anticipation of retaliatory action by PIJ – this gave the IDF time to move military assets into place, in preparation for today’s operation.
Israel says attempts to get PIJ to climb down have been exhausted, and so they’ve now decided to act first and launched “Operation Breaking Dawn”.
Strikes on a high-rise building killed a number of PIJ militants, including the commander of Islamic Jihad in northern Gaza. A five-year-old child was also among the dead, and more than 40 were wounded.
Israeli officials are making it clear this was a strike against a PIJ target, not against Hamas, the militant group that controls Gaza. That message has been passed onto Hamas via backchannels.
Palestinian Islamic Jihad has already said it will respond and warned that Tel Aviv, just north of Gaza, will be targeted.
Hamas, which Israel says it wants to keep out of this, must now decide what it does next. If the group joins forces with PIJ and fires rockets into Israel, then things could escalate very quickly, and we could be looking at a second war in as many years.
Senior IDF commanders have hinted to me that their objectives have been achieved, however they warn of further airstrikes if they receive intelligence that preparations are under way for attacks on Israel.
Much hangs on the next 24 hours.
Israeli Defence Minister Benny Gantz visited communities near Gaza earlier on Friday, saying authorities were preparing “actions that will remove the threat from this region”, without elaborating.
“We will operate with internal resilience and external strength in order to restore routine life in Israel’s south,” he said.
“We do not seek conflict, yet we will not hesitate to defend our citizens, if required.”
The PIJ is smaller than Hamas but shares a similar ideology, with both groups opposing Israel’s presence.