Hamas has fired multiple rockets into Israel hours after six fighters died during Israel's first major incursion into the Gaza Strip since June's truce.
Israel said 35 rockets and mortars were fired, but gave no word on casualties.
Troops had entered Gaza to destroy what Israel said was a tunnel dug by militants to abduct its troops.
One militant died in the gunfight, and a subsequent Israeli air strike on Hamas positions in southern Gaza killed at least five fighters, medics said.
An Israeli military spokeswoman said three air strikes took place, targeting militants who had fired mortars and rockets at Israeli forces.
The fighting broke out on Tuesday evening as Israeli tanks and a bulldozer moved 250m into the central part of the coastal enclave, backed by military aircraft, says the BBC's Aleem Maqbool in Ramallah.
Residents of central Gaza's el-Bureij refugee camp said a missile fired from an unmanned Israeli drone flying over the area injured another three Hamas gunmen.
A truce between the two sides had so far largely held since it was declared on 19 June, although according to the Israeli military dozens of rockets and missiles have been fired in the past four and a half months.
Israel said Tuesday's raid was not a violation of the ceasefire, but rather a legitimate step to remove an immediate threat.
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