Iran’s supreme leader, Ayatollah Ali Khamenei, said on Wednesday that abandoning uranium enrichment was “100 percent” against Tehran’s interests, effectively rejecting a key U.S. demand in weeks of tense negotiations over Iran’s nuclear program.

Iran’s foreign minister said Tuesday that Tehran will respond to a recent US proposal on the Iranian nuclear program “in the coming days.” “The US proposal contains many ambiguities and questions,” Abbas Araghchi said during a book signing event in the Lebanese capital, Beirut.

Iran and the United States are set to begin a fifth round of high-stakes nuclear talks in Rome on Friday amid growing skepticism in Tehran about the chances of a deal as Washington hardens its position.