The United States is skeptical of reports suggesting Iran had developed a hypersonic missile, a Pentagon spokesperson said on Thursday.

The semi-official Tasnim news agency last week quoted the Revolutionary Guards’ aerospace commander as saying Iran had built a hypersonic ballistic missile.

“We’ve seen the reports asserted and coming out of Iran, we remain skeptical of these reports,” Sabrina Singh told a news conference.

Hypersonic missiles can fly at least five times faster than the speed of sound and on a complex trajectory, making them difficult to intercept.