Israel's supreme court ruled Thursday that it was illegal not to return the bodies of dead Palestinian assailants to their families unless an exchange was agreed.

Current legislation does not authorise the refusal to return the bodies, the court said, following a motion filed by families of nine Palestinians.

However, the Jewish state's highest court granted the government six months to pass a law allowing it to keep hold of the corpses, otherwise the bodies would have to be handed over.

The government announced last year that it would not return the bodies unless the Palestinians agreed to an exchange for the remains of two Israeli soldiers believed to have been killed in a 2014 war in Gaza.

Prime Minister Benjamin Netanyahu indicated last month that Israel would use the bodies of Palestinian militants killed in a Gaza tunnel explosion as part of a deal to receive those of two Israeli soldiers.

On Thursday, Netanyahu described the court's ruling as "very problematic".

"Hamas must not be offered free gifts, and I will convene a meeting on Sunday... in order to find a concrete and legal solution to keep putting pressure on Hamas," he said on his official Facebook page.