Iran's top leader has hardened his stance in nuclear negotiations with world powers as a deadline for a final deal rapidly approach, saying he rejects a long-term freeze on nuclear research and wants to ban international inspectors from accessing military sites.

The comments by Ayatollah Ali Khamenei, who repeatedly has backed the Islamic Republic's negotiators amid criticism from hard-liners, may give his diplomats little room for concessions ahead of the June 30 deadline. They also directly challenge the U.S., especially his demand that Iran only will sign a final deal if economic sanctions are first lifted.

Iran's parliament already has passed a bill that, if ratified, will ban access to military sites, documents and its scientists as part of any future deal. The bill must be ratified by the Guardian Council, a constitutional watchdog, to become a law.

Speaking Tuesday night in comments broadcast on Iranian state television, Khamenei called demands Iran halt the research and development portion of its nuclear program "excessive coercion."

"We don't accept 10-year restriction. We have told the negotiating team how many specific years of restrictions are acceptable," Khamenei said. "Research and development must continue during the years of restrictions."

Khamenei accused the U.S. of offering a "complicated formula" for lifting sanctions. He added waiting for the U.N.'s International Atomic Energy Agency to verify its cooperation would take too long.

"Lifting sanctions can't depend on implementation of Iran's obligations," he said.

Khamenei also said he rejects any inspection of military sites or allowing its scientists to be interviewed. Iran's nuclear scientists have been the targets of attacks before both inside the Islamic Republic and elsewhere.

The U.S.' "goal is to uproot and destroy the country's nuclear industry," he said. "They want to keep up the pressure and are not after a complete lifting of sanctions."

In a statement Sunday, the U.S. State Department said inspections remain a key part of any final deal.

Tehran is negotiating with the U.S., Russia, China, Britain, France and Germany over its contested nuclear program. The talks are focused on reaching a final accord that curbs Iran's nuclear program in return for the lifting of economic sanctions.

Iran says its nuclear program is for peaceful purposes, such as power generation and medical research. The West fears Iran could use it to finally build an atomic bomb.

Negotiations likely will begin in earnest in the coming days in Europe. On Wednesday, Iran's official IRNA news agency reported that deputy foreign ministers Abbas Araghchi and Majid Takht-e-Ravanchi had resumed talks with Helga Schmidt, a deputy of European Union foreign policy chief Federica Mogherini. It did not elaborate.