Republican presidential hopeful Marco Rubio penned a letter Friday blasting Secretary of State John Kerry for failing to negotiate the release of four Americans who are currently imprisoned in Iran.

Rubio, who is a member of the Senate Foreign Relations Committee, has been a staunch critic of the nuclear deal with Iran since it was unveiled by President Obama on Tuesday. He has repeatedly accused the Obama administration of negotiating from a position of weakness and recently faulted the president and Kerry for "giving concession after concession to a regime that has American blood on its hands."

In his second letter to Kerry, the Florida senator says it is "unacceptable that the United States has reached a final agreement with Iran while innocent Americans languish in the most brutal conditions of Iranian jail cells."

He notes that just 24 hours before the administration revealed its agreement with Iran, a Washington Post reporter named Jason Rezaian "was subjected to a closed-door hearing" where only his Iranian lawyer were allowed in the courtroom. Rezaian, who is an American citizen and the Washington Post's Tehran correspondent, was arrested about a year ago by Iranian officials who accused him of espionage.

Rezaian is one of four U.S. citizens behind bars in Iran who Rubio and other lawmakers have accused the administration of abandoning.

Other American prisoners include Pastor Saeed Abedini, who was handed an eight-year prison sentence after being accused of subverting Iran's national security by establishing several Christian churches throughout the country; Amir Hekmati, a former marine who was sentenced to death for espionage and then successfully appealed only to receive a 10-year sentence for aiding a hostile nation; and former FBI Agent Robert Levinson. who was taken hostage while visiting Iranian territory in 2007 and has been missing ever since.

"I am disturbed by how the administration has missed an opportunity to make the freedom of these Americans a priority in your negotiations with Iran," Rubio wrote to Kerry. "Despite your claims that these negotiations were only about Iran's nuclear program, Iran successfully obtained non-nuclear concessions from these negotiations that will aid its efforts to sow terror and instability throughout the Middle East."

Rubio's most recent letter to Kerry is the second time he and other GOP leaders have placed a spotlight on the Americans imprisoned in Iran. In March, he joined 19 Republican senators who urged Kerry to "discuss [the] imprisoned Americans and demand their unconditional release as you engage in discussion with Iranian officials."

This time around, Rubio is calling on Kerry to "use every tool at [his] disposal to secure [the imprisoned Americans'] freedom."