Chief of General Staff Lieutenant General Dan Halutz said there was an ongoing investigation into the artillery shelling of the beach, where Palestinian families were relaxing.
The investigation has so far ruled out the fire resulted from a gunship or an airstrike, but the possibility remains that artillery rounds were fired, Halutz told reporters.
There is also the possibility that the incident was caused by Palestinian militants, who accidentally misfired one of their rockets, he said.
"First I want to express our - my - regret over the incident where uninvolved civilians were harmed," said the general.
"But I stress that the expression of regret does not mean we are taking responsibility for the incident.
"The ... investigation has not yet been completed."
Chief Halutz emphasised that the shelling was preceded by a series of Kassam rockets that were fired into Israel.
The first phase of the military investigation has concluded that the explosion was caused either by an accidental shelling or by an internal Palestinian incident.
Commander of Gaza Division, Brig.-General Aviv Kochavi said there was mounting evidence supporting the latter option.
Meanwhile, hundreds of demonstrators gathered outside the General Halutz's home near Tel Aviv to protest against the beach shelling.
Demonstrators held banners reading: "you have a murderer in your neighbourhood".
The Palestinian President, Mahmoud Abbas, has declared three days of mourning for the victims, five of whom were members of a single family.
He described the shelling as one of "Israel's "bloody
massacres".
The military wing of the Islamic militant group Hamas has ended its 18-month truce with Israel.
It says it has fired at least 14 rockets into Israel in response to the shelling.