TEHRAN, Iran — 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.

Ayatollah Ali Khamenei endorsed the deal in a letter to moderate President Hassan Rouhani that was read on state TV on Wednesday. Until now he had publicly declined to approve or reject the deal while expressing support for Iran’s negotiators. Khamenei has the final say on all major policies.

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.