The United States and Iran are set to begin a third round of nuclear talks this weekend, entering what experts describe as a more difficult phase of technical negotiations as Washington lays out stricter conditions.

Iran sees its missile programme rather than its uranium enrichment as a bigger obstacle in nuclear talks with the United States that resume on Saturday, an Iranian official with knowledge of the talks said on Friday.

Negotiations between Iran and the U.S. over Tehran’s rapidly advancing nuclear program will proceed Wednesday.