The U.S. and Iran have submitted multiple peace proposals since Pakistan brokered a ceasefire between the two sides on April 8 that U.S. President Donald Trump maintains is still in effect even as the countries exchanged fire.

On Friday, U.S. Secretary of State Marco Rubio said the U.S. expected Iran’s proposal over the weekend, adding “the hope is it’s something that can put us into a serious process of negotiation.”

Also on Friday, the U.S. military said its forces struck and “disabled” two Iranian-flagged oil tankers to stop them pulling into an Iranian port on the Gulf of Oman. The attacks on the ships near the Strait of Hormuz cast more doubt on the tenuous ceasefire.

“U.S. forces in the Middle East remain committed to full enforcement of the blockade of vessels entering or leaving Iran,” U.S. Central Command commander Brad Cooper said in a written statement.

Trump has warned that the U.S. would “knock [Iran] out a lot harder, and a lot more violently,” if Tehran does not accept a peace deal.

Iranian Foreign Minister Abbas Araghchi on Friday accused Washington of opting for a “reckless military adventure” every time a “diplomatic solution is on the table.”