Iranian leaders have recently agreed to negotiate “aspects” of their nuclear program, Secretary of State Marco Rubio revealed on Tuesday.While he acknowledged there’s no guarantee their willingness to negotiate on parts of the nuclear question means the two sides will be able to finalize an agreement on it, the secretary said.“For the first time, certainly in my memory, they have agreed to negotiate aspects of their nuclear program that just a month ago, just a year ago, they were refusing to even mention, much less enter into discussions about. That is not a guarantee that ultimately will lead to a deal that’s acceptable to the Senate or acceptable to the American people, but we’ll be able to engage them in a process to truly test the proposition of how far they’re willing to go,” he said.

Rubio, who testified in front of the Senate Foreign Relations Committee on Tuesday morning, did not specify which aspects of its nuclear program the country had expressed a willingness to negotiate.

The Trump administration is currently pursuing a phased agreement with Iran, the first portion of which would focus on the Strait of Hormuz. The strait is a narrow body of water that connects the Persian Gulf to the Gulf of Oman, and roughly 20% of the world’s oil would transit the waterway on a daily basis prior to the conflict, but at the conflict’s onset, Iranian forces threatened to attack commercial vessels and placed mines in the water.