The Prime Minister has spoken to his Israeli counterpart Benjamin Netanyahu about the situation in Gaza and Southern Israel.
He said that "rocket attacks from Gaza into southern Israel by Hamas were completely unacceptable."
A Downing Street spokesperson said:
The Prime Minister said that he was extremely concerned by the dangerous situation and deeply sorry for the loss of civilian life on both sides.
He said the rocket attacks from Gaza into southern Israel by Hamas and other armed groups were completely unacceptable and that the increasing frequency of rocket attacks in recent days was the immediate cause of the situation.
He made clear that Hamas bears the principal responsibility for crisis.
"The Prime Minister said that the priority must be to de-escalate the crisis.
He called on Mr Netanyahu to do all he could to avoid civilian casualties and emphasised that both sides needed to avoid a spiral of violence that would be in no-one's interest, particularly at a time of instability in the region.
Number 10 says the PM hopes to speak to the Egyptian president about the same issue tomorrow.
Today's bombing in Tel Aviv is reminiscent of six years ago and could mark a serious escalation in the conflict.
A truce in Gaza suits all sides but none of the groups seem interested in a long-lasting solution.
A complex system of air defences has been instrumental in protecting Israel's population from incoming rockets.