"Israelis have to understand that while they are defending their security in seeking to root out these rocket launchers and deal with the attack tunnels, they are also undermining the support for Israel that exists in the west," said Britain's Foreign Minister Philip Hammond.
Speaking on BBC Radio 4's Today programme, Hammond, who was in Israel last week, also said: "What Israel does in Gaza must be proportionate - that's a requirement of international law. It would not be legal if it was not proportionate."
"Israel will argue that the actions it has taken are taken against military targets and that Hamas has deliberately planted military installations in the middle of civilian areas, using civilians as human shields. In due course, the claims on both sides will need to be investigated. What is needed right now is an immediate and unconditional ceasefire."
The British government has repeatedly emphasized Israel's right to defend itself but called for an end to the violence.