Gaza based channels reported spotting IDF tanks at the Netzer intersection in Gaza City, a strategic point deep inside the enclave.
More than 100,000 Palestinians have fled south over the past two days, according to Israel, but they still face bombardment and dire conditions.
Sound of explosions broke the 72-hour lull in Gaza on Friday after Hamas refused to extend the ceasefire and fired a barrage of rockets at Israel which responded by air strikes killing a child, as Egypt-mediated talks to negotiate a durable truce hit a deadlock.
The advance of the Al-Qaeda splinter group the Islamic State in Iraq and Syria and its allies towards Baghdad appears to have been "stalled" but the "grave" situation in Iraq needs to be dealt with militarily as well as politically, a top UN official has said.