Barak criticized a provision in a draft agreement in which three-quarters of Iran's stockpile of nuclear fuel would be sent to Russia to be enriched enough for use in a nuclear reactor but not in a weapon.
Speaking at a presidential conference in Jerusalem, Barak urged other countries to set a deadline for Iran to halt its nuclear program and impose more sanctions, ynetnews.com reported.
"The uranium enrichment by Iran must be stopped altogether," Barak said. "Iran received legitimization for enriching uranium for civilian purposes on its soil, contrary to the understanding that those negotiating with it have about its real plans -- obtaining nuclear (weapons) capability."
The United States, France, Russia and Iran have until Friday to approve the draft International Atomic Energy Agency deal on Iran's nuclear fuel.
Negotiators had been meeting in Vienna the past several days, trying to resolve Iran's nuclear fuel issue before the tentative agreement was struck Wednesday, the United Nations said.