US Secretary of State Rex Tillerson said Wednesday he had had a matter of fact exchange with his Iranian counterpart Mohammad Javad Zarif but that Washington continues to have "significant issues" with the Iran nuclear deal.

"It was a good opportunity to meet, shake hands. The tone was very matter of fact. There was no yelling, we didn't throw shoes at one another," Tillerson said.

"It was not an angry tone at all. It was a very, very matter of fact exchange about how we see this agreement very differently."

In particular, Tillerson said that US President Donald Trump finds the so-called "sunset clause" in the 2015 deal -- which sees the restrictions on Iran's nuclear enrichment lifted after 2025 -- to be "unacceptable."