U.S. forces struck two empty Iranian-flagged oil tankers in the Gulf of Oman on Friday, escalating pressure on Tehran’s shipping network even as Washington says a potential deal to halt the war could be close.

The president didn’t offer any details about the potential deal or what he expected Iran to give, but warned of a resumption of strikes if the agreement fell through.

The attack occurred as Iran and the U.S. are officially in a ceasefire.