Iran's Supreme Leader Ayatollah Ali Khamenei on Monday welcomed the lifting of international sanctions against Iran, but warned that Tehran should remain wary of its old enemy the United States.State television reported that Khamenei wrote to President Hassan Rouhani to congratulate him on implementing the nuclear deal, which resulted in US, 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."Be careful that the other side fully meets its commitments. The comments made by some American politicians in last two, three days are suspicious," he added.Republican candidates for the US presidency have criticized the deal, and some Iranian officials fear Washington could walk away from the deal when President Barack Obama leaves office in early 2017.Hopes for a broader rapprochement between the two countries were dashed on Sunday when Washington slapped new sanctions on companies accused of supporting Iran's ballistic missile program, drawing an angry response from Iranian officials.