"I am horrified by last night's Israeli airstrikes and shelling in Gaza," Volker Turk said in a statement, adding that "This will add tragedy onto tragedy."

Israel on Tuesday vowed to continue fighting in Gaza until the return of all hostages as it unleashed its most intense strikes since a truce took effect on January 19.

The health ministry in the Hamas-run territory reported more than 330 people killed in the latest strikes.

Hamas, which has not responded to the strikes so far, accused Israeli Prime Minister Benjamin Netanyahu of deciding to "resume war" after an impasse in truce negotiations, and warned that the return to fighting could be a "death sentence" for hostages still alive in Gaza.

"This nightmare must end immediately," Turk said, adding that "the last 18 months of violence have made abundantly clear that there is no military path out of this crisis".

"The only way forward is a political settlement, in line with international law. Israel's resort to yet more military force will only heap further misery upon a Palestinian population already suffering catastrophic conditions," he said.

"The hostages must be released immediately and unconditionally. All those arbitrarily detained must be released immediately and unconditionally. The war must end permanently," Turk said.

"We urge all parties with influence to do all in their power to achieve peace and avoid further suffering of civilians."