Six Palestinians, including several militants, have died in a series of Israeli strikes in the Gaza Strip.
Israel says its targets are militants who pose a threat to its forces
An Israeli army spokesman said troops had carried out air and ground attacks against "armed militants" around Khan Younis in the southern Gaza Strip.
But Palestinians said two of the dead were civilians, including a 60-year-old woman, and that 26 people were injured.
Troops moved into Gaza in one of the biggest incursions since Hamas seized control of the strip in mid-June.
Palestinian reports speak of about 20 tanks and armoured personnel carriers being involved in the operation.
An Israeli army spokesman said its aim was to stop militants digging tunnels and firing rockets into Israel.
About 100 Palestinians had been rounded up for questioning, the army said.
The Hamas takeover of Gaza has left the Palestinian territories split and under the control of the rival factions - Hamas in Gaza, and Fatah in the parts of the West Bank not controlled by Israel.
Israel and the international community have backed Palestinian Authority President and Fatah leader Mahmoud Abbas, while increasing the isolation of Hamas and completely cutting off Gaza and its 1.5 million residents.