The United States on Tuesday said that Hamas had chosen war by refusing to release hostages, as Israel unleashed its most intense strikes since a ceasefire, killing more than 400 people, and vowed to continue fighting until all hostages are returned.

The US National Security Council spokesman Brian Hughes said in a statement that "Hamas could have released hostages to extend the ceasefire but instead chose refusal and war".

Hughes' statement comes as Hamas leader Sami Abu Zuhri said that the Israeli strikes in the Gaza Strip were an attempt to force the group's "surrender", and called the United States "complicit" in the escalation.

"The aim of the massacres committed by the occupation in Gaza is to undermine the ceasefire agreement and attempt to impose a surrender agreement, writing it in the blood of Gaza," he said in a statement.

But Israel's foreign minister Gideon Saar said that his country had "no alternative" other than to resume military operations in Gaza after efforts to secure the release of hostages failed.

Saar's comments come as relatives of the Israeli hostages accused the country's prime minister Benjamin Netanyahu of sacrificing their loved ones by carrying out the strikes.

They called on supporters of the hostages to demonstrate outside Netanyahu's office in Jerusalem, warning that "military pressure could further endanger their lives and complicate efforts to bring them back safe and sound".

Liran Berman is a brother of twin siblings who are being held hostage in Gaza.

"Right now my brothers are alive. My worst nightmare is to become a family of deceased and killed hostages. This is my worst nightmare right now," he said.

Meanwhile, the British government has called on Israel and Hamas to implement their ceasefire agreement for Gaza in full, and the EU has called for an immediate end to the renewed violence in the strip.

And Moscow has said it deeply regrets Israel's resumption of military operation in Gaza. (Agencies)