VATICAN CITY - Relations between the Vatican and Israel grew tense on Thursday when the Jewish state condemned an aide to Pope Benedict for calling Gaza "a big concentration camp."

Israel criticised Cardinal Renato Martino as the pope delivered a speech to diplomats in which he spoke out against the use of violence by both Israel and Hamas Islamists in Gaza.

On Wednesday, Martino, president of the Vatican's Council for Justice and Peace, delivered the Vatican's toughest criticism of Israel since its offensive in the Palestinian-ruled enclave, calling Gaza a "big concentration camp."

More than 750 Palestinians have been killed since Israel started bombarding Gaza on December 27 with the aim of halting Hamas rocket attacks. At least eleven Israelis have been killed, eight of them soldiers, since the offensive began.

In a follow-up interview in Italy's La Repubblica newspaper on Thursday, Martino defended his comments, saying the people of Gaza "are surrounded by a wall that is difficult to breach, in conditions that go against human dignity."

Martino said "certainly, the rockets of Hamas are not confetti. I condemn them" but forcefully criticised Israel for an attack on a U.N. school.