U.S. Senator John McCain on Sunday threatened to block a potential nuclear deal with Iran.
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"We will do everything in our power to make sure we do play a role," McCain, the chairman of the Senate Armed Services Committee, told CNN.
McCain was one of 47 Republican senators who sent a letter to the Iranian leadership saying the Senate must approve any nuclear deal.
"We will insist on approval," McCain said. "I don't think that they will reach a deal that we can live with."
The United States, Britain, China, France, Russia and Germany are pushing to agree on the outlines of a nuclear deal with Iran by the end of the month and a detailed agreement by the end of June.
The deal would place strict long-term limits on Iran's civilian nuclear program, making it harder for Iran to use nuclear materials to make nuclear weapons. International sanctions would be lifted in return.
The U.S. Congress could derail a deal by pursuing new sanctions against Iran or by refusing to lift those already in place. It remains to be seen if the Republican-led legislature has enough votes to succeed in either route, and U.S. President Barack Obama has ultimate veto power.
Also on Sunday, CIA Director John Brennan told Fox News that regardless of whether a deal is struck, more pressure must be brought to bear on Iran both to halt its nuclear aspirations and to curb its efforts as a state sponsor of terrorism.
"What we have to do, whether there's a deal or not, is continue to keep pressure on Iran and to make sure that it is not able to continue to destabilize a number of the countries in the region," Brennan said.