Philip Hammond, the foreign secretary, has said Israel is undermining its support in the west but has refused to say whether the country's shelling of Gaza is proportionate.

The senior Tory, who took over the brief in this month's reshuffle, said proportionality was an emotive word and claims on both sides would have to be investigated.

He repeated David Cameron's calls for an immediate humanitarian ceasefire after another heavy night of shelling in which a school was reportedly hit and many people died.

Speaking on BBC Radio 4's Today programme, he said: "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."

The government has repeatedly emphasised Israel's right to defend itself but called for an end to the violence.

Ed Miliband has done the same but clearly opposed Israel's incursion into Gaza, despite "extreme provocation from Hamas".

Nick Clegg, the Liberal Democrat leader, has gone even further by saying the Israeli response "appears to be deliberately disproportionate".

Pressed on his views about Israel's proportionality, Hammond refused several times to be drawn.

"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."