Nick Clegg has come close to accusing Israel of breaching international law after he said its response to the Hamas rocket attacks from Gaza was "deliberately disproportionate".

In a marked contrast with David Cameron, who has moderated his criticism of Israel, the deputy prime minister accused Israel of imposing a "disproportionate form of collective punishment" on citizens in Gaza.

Clegg, who spoke out after Israeli shells killed four children on a beach in Gaza, said that Israel had the right to defend itself as Hamas launches a series of rocket attacks.

But speaking on his weekly LBC phone-in, he said that Israel had gone too far. In remarks which echoed language used by the International Committee of the Red Cross (ICRC) to define breaches of international law – and even war crimes – Clegg said: "I have to say though I really do think now the Israeli response appears to be deliberately disproportionate. It is amounting now to a disproportionate form of collective punishment. It is leading to a humanitarian crisis in Gaza, which is just unacceptable.

"I really would now call on the Israeli government to stop. They have proved their point. Israeli of course retains the right to react. But you cannot see the humanitarian suffering in Gaza now and the very great number of deaths in Gaza without concluding that there is not much more going to be served in Israel's own interests … to see this festering humanitarian crisis get worse. It incubates the next generation of violent extremists who want to do harm to Israel."

Lib Dem sources stressed that Clegg was making a political, rather than a legal, argument. But his reference to a "deliberately disproportionate" response by Israel echoes the analysis by the ICRC. A paper defining war crimes includes a section which says: "Both indiscriminate and disproportionate attacks can be likened to attacks on civilians if the perpetrator was aware that this would be the effect of the attack in the ordinary course of events."

The ICRC also says that "collective punishment" is illegal and a war crime. It says: "Collective punishments are prohibited by Additional Protocol II and customary international law … In addition, collective punishments constitute a war crime because they consist of the deprivation of the right to fair trial and may also constitute cruel treatment."

Clegg said he hoped the humanitarian ceasefire would help ease the current attacks.

"Today we have the glimmer of hope that a five-hour humanitarian ceasefire has been entered into by both sides and my plea today to both sides is: please build on that. Further deaths, more violence begetting more violence, is not in anybody's interests. It is not going to help deliver the only way in which Israelis will be able to live in security and peace in the long run which is a negotiated two state peace settlement."

The deputy prime minister spoke of "truly unimaginable humanitarian suffering in Gaza". He said: "Regardless of what side you are on this ancient, bloody conflict no one can feel indifferent to the spectacle of this overcrowded, desperate sliver of land, Gaza, where so may thousands of people are suffering."

But Clegg stressed that Israel had the right to defend itself. He said: "I will always defend Israel's right to respond and to defend itself in the face of violence that is designed to terrorise Israeli citizens.

"I have spoken out repeatedly about Israel's very legitimate demands that Hamas and other recognise Israel's right to exist and to exist peacefully within its own borders and to provide security to its ow citizens."