Iran’s Supreme Leader Ayatollah Ali Khamenei gestures as he delivers a speech during a gathering by Iranian forces.

Iran’s supreme leader has endorsed a landmark nuclear deal reached with world powers, but has warned the government to be vigilant, saying the United States “cannot be trusted,” in a letter published on his website.

Ayatollah Ali Khamenei, who has the final say on all major policies in Iran, endorsed the deal in a letter addressed to moderate President Hassan Rowhani that was read on state TV on Wednesday. He said Iran could go ahead but “ambiguities” in the agreement must be guarded against.

Khamenei also said U.S. President Barack Obama has sent him two letters assuring him that the U.S. had no intention to topple Iran’s clerical establishment.

Khamenei said he had approved the decision of the country’s top security committee, the Supreme National Security Council, to implement the nuclear deal but it must be “tightly controlled” and monitored “to prevent significant damage” to the country’s future.

His endorsement comes after his long-absent public approval or rejection of the deal while expressing support for Iran’s negotiators.

The agreement reached in July with the U.S., Britain, France, China, Russia and Germany would curb Iran’s nuclear activities in exchange for the lifting of crippling international sanctions.

Western nations have long suspected Iran of pursuing nuclear weapons alongside its civilian program, charges rejected by Tehran, which insists its program is entirely peaceful.

Iran won’t give up uranium stockpile

In the same time, Khamenei said Iran will not give up its stockpile of enriched uranium or repurpose the heavy water nuclear reactor at Arak until the issue of possible military dimensions (PMDs) to its disputed nuclear program is solved.

“Any action regarding Arak and dispatching uranium abroad … will take place after the PMD file is closed,” he said.

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