Iran's Supreme Leader Ayatollah Ali Khamenei welcomed the lifting of international sanctions against Iran, but warned that Tehran should remain wary of its old enemy the United States.

According to Reuters, a state television report said that Khamenei wrote to President Hassan Rouhani to congratulate him on implementing the nuclear deal, which resulted in U.S., European Union and United Nations sanctions being lifted over the weekend.

In his first comments since the deal took effect, Iran's highest authority made clear that Washington should still be treated with suspicion. He made no mention of a surprise prisoner exchange that also took place this weekend.

"I reiterate the need to be vigilant about the deceit and treachery of arrogant countries, especially the United States, in this (nuclear) issue and other issues," Khamenei said.