Gaza crisis: Israeli air strikes hit Hamas HQ
Israel has targeted the headquarters of Hamas leaders and other key facilities in Gaza, on the fourth day of Israeli air strikes in the territory.
Prime Minister Ismail Haniya's office, which Egypt's PM had visited on Friday, was among the buildings destroyed.
At least 39 Palestinians and three Israelis have died since Israel killed Hamas's military chief on Wednesday.
Israel earlier put 75,000 reservists on stand-by amid speculation of an impending ground invasion.
Militants in Gaza have continued to fire rockets into Israel.
Sirens went off around Tel Aviv again on Saturday, with Israel's military saying that a missile had been intercepted by its Iron Dome defence system.
Earlier, Gaza City was hit by a string of large explosions shortly after 03:00 (01:00 GMT).
There was another series of strikes in and around the city after 05:00, with several targeting Hamas's cabinet buildings, which correspondents say were likely to have been empty.
Another of the targets was the house of a Hamas leader in Jabaliya, north of Gaza City.
The BBC's Paul Danahar tweeted from the scene: "A mother in her wrecked home... is scurrying around collecting her daughter's dolls, dusting them off."
Our correspondent said Mr Haniya's HQ was the most damaged of any building he had seen. Egyptian Prime Minister Hisham Qandil had visited it on Friday morning.
At least nine Palestinians are reported to have been killed in strikes since Friday evening. They are said to include three members of the Izz al-Din al-Qassam Brigades, Hamas's military wing.
Israel said it was targeting rocket launchers, weapons storage facilities and smuggling tunnels on the border with Egypt in southern Gaza.
Israeli military spokeswoman Avital Leibovich said 200 targets had been hit overnight. The army told the BBC it wanted to hit hundreds more and that it was legitimate to target anything connected with Hamas.
There are rumours that a ground attack is imminent, but Israeli officials say no decision has been made.
Government spokesman Mark Regev told the BBC the operation would end when Israeli citizens were safe, and that all options - including a ground incursion - remained "on the table".
Israel blocked access to three major routes leading into Gaza on Friday. Call-up papers have already been sent to 16,000 Israeli reservists, with officials authorising the mobilisation of another 75,000.
Militants and civilians, including at least seven children, have been among the Palestinians killed during Israeli strikes in recent days, Hamas says.
The group's military leader Ahmed Jabari was killed on Wednesday. A senior commander was killed on Friday, officials said.
Before the recent offensive - codenamed Pillar of Defence - Israel had repeatedly carried out air strikes on Gaza, as Palestinian militants fired rockets across the border.
Hundreds of rockets have been fired into Israel from Gaza since Wednesday.
Most of the hits were in the south, but a small number have been aimed at Tel Aviv and Jerusalem. The three Israelis who died were in a building in the southern town of Kiryat Malachi which was hit by a rocket on Thursday.
A quarter of the attacks have been intercepted by the Iron Dome system, officials say.
Dozens more rockets were fired from Gaza on Saturday morning. Several landed in the southern city of Ashdod, damaging buildings.
Three Israeli soldiers were injured in another strike in the south, the military said. A new Iron Dome battery has been deployed in the Tel Aviv area.
Palestinian Authority President Mahmoud Abbas has accused Israel of carrying out "massacres".
Tunisian Foreign Minister Rafik Abdessalem arrived in Gaza through the Rafah border crossing from Egypt to show support for Hamas. Later on Saturday he visited the wreckage of Mr Haniya's HQ.
Western leaders and UN Secretary General Ban Ki-moon have appealed for both sides to stop the violence.
In a phone conversation with Israeli Prime Minister Benjamin Netanyahu on Friday, President Barack Obama reiterated US support for Israel's "right to defend itself".
Mr Obama also spoke to Egyptian President Mohammed Mursi. Mr Mursi has called the Israeli raids "a blatant aggression against humanity" and promised that Egypt "will not leave Gaza on its own".
Ties between Hamas and Egypt have strengthened since Mr Mursi's election earlier this year.
Hamas was formed as an offshoot of Mr Mursi's Muslim Brotherhood.