At least eight people have died, including a senior militant, as a result of Israeli airstrikes, Palestinian 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 boy 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 League) 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.
A Palestinian Islamic Jihad official named Tayseer al-Jaabari as the senior commander killed in the strikes.
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.