Hamas bears "principal responsibility" for the escalating crisis between Gaza and Israel, Downing Street said tonight.
Number 10 also called rocket attacks on Israel by Hamas "completely unacceptable".
It said Prime Minister David Cameron was "extremely concerned" by the escalation in violence.
Number 10 said Mr Cameron spoke to Israeli prime minister Benjamin Netanyahu tonight by telephone to discuss the situation following Israel's assassination of Ahmed Jabari, the Hamas military chief, in a drone strike on his car yesterday.
"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," they said.
A Downing Street spokesman said: "The Prime Minister spoke to the Israeli Prime Minister, Benjamin Netanyahu, this evening to discuss the situation in Gaza and Southern Israel.
"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."
Downing Street said the Prime Minister hopes to speak to the president of Egypt, Mohammed Morsi, tomorrow on the same issue.