At least nine Palestinians have been killed amid Israeli operations in Gaza, witnesses and medics say.
Schoolchildren were reported injured by Israeli tank fire
Several of the deaths occurred during raids by Israeli troops east of Khan Younis in the early hours of Thursday. Civilians were among those killed.
The fighting is some of the heaviest seen in recent weeks.
Israel says it is trying to stop rocket fire into Israel. A rocket launched on Thursday reached the city of Ashkelon, some 17km (10 miles) north of Gaza.
No-one was hurt, but the Israeli army said it was the furthest a rocket launched from Gaza had reached.
Israeli spokesmen said greater Palestinian rocket range was putting a far greater number of Israelis in the firing line.
Israeli tanks and troops moved into the area close to Khan Younis early on Thursday, sparking exchanges of fire with militants, reports said.
In the deadliest single attack, a tank shell struck a house, killing five members of the same family.
Two have been claimed by the militant group, Islamic Jihad, as its fighters. The other three, including a mother and daughter, were civilians, medical sources say.
Israeli troops are later reported to have become trapped in one area, prompting air strikes to be launched in an attempt to rescue them.
At least two people died in those strikes, said medics.
Subsequent tank fire injured at least three schoolchildren, said witnesses and doctors.
One 14-year-old boy was left in critical condition, the Associated Press news agency said.
In a separate exchange of fire in Gaza City, Hamas reported one of its gunmen dead.
Israeli troops have also made an incursion into Nablus in the West Bank, and there have been clashes in the centre of the city, reports the BBC's Aleem Maqbool.
At least 19 people have been injured, mostly young stone-throwers hit by rubber bullets, he says.
The Israeli incursion follows similar pre-dawn strikes in Gaza on Wednesday which killed at least six.
It comes days before US President George W Bush visits the region to try to promote a peace agreement, in the last year of his administration.
Gaza is run by Hamas, which took over the territory last June after a violent struggle with its Fatah rivals.
Hamas is deemed a terrorist group by the US, and was not invited to the Annapolis conference organised by Washington in November, which kick-started the renewed attempt to agree a peace deal.