The escalation came shortly after Israeli defence minister warned that an extensive Israeli military operation was only a matter of time.
Late on Wednesday, nine Palestinians, two of them civilians, were killed in two separate Israeli strikes across the impoverished territory, Palestinian officials said.
Twenty-five Palestinians were also wounded, hospital officials said, in what was the bloodiest day in the Gaza Strip since Israel declared it a "hostile territory".
An air strike on a car in Gaza City on Wednesday left at least five people dead, while Israeli shelling killed four Palestinians in the northern Gaza Strip, witnesses said.
Mortuary officials said those killed in the attack on the car were members of the armed group Army of Islam. The group was involved in the kidnapping of Alan Johnston, a BBC reporter, in March.
An Israeli military spokeswoman confirmed there had been an attack by the air force, but had no word on its target.
Meanwhile, Israeli shelling killed four people in the northern town of Beit Hanoun, according to witnesses, and wounded a large number of others.
Israeli operation
Israel said it was responding to ongoing rocket and mortar fire from Gaza at Israeli border towns.
Earlier, Palestinian fighters in Gaza said they have fired more than 12 rockets at the southern Israeli town of Sderot.
Israeli tanks took control of Beit Hanoun, from where some of the rockets had been fired.
Ehud Barak, the Israeli defence minister, said there could soon be a broad military move on Gaza in an attempt to stem the cross-border rocket fire.
"We are getting closer to carrying out a widespread operation in Gaza which, for many reasons, has not taken place in the past weeks," Barak told Israel's Army Radio.
Nour Odeh, Al Jazeera's correspondent in Gaza, said: "Residents of the Gaza Strip are anticipating the worst."
She said that following Israel's declaration of Gaza as "hostile territory", "people are perceiving these air strikes and incursions as a translation of that declaration".