Gaps between the two sides include how quickly Western nations would lift punitive sanctions against Iran, and what types of nuclear research Tehran would be allowed to continue, diplomats said. Negotiators also are grappling over access by international inspectors to Iran’s military sites, where evidence has suggested the possibility of past experiments on weapons technology.

The differences between Iran and international powers are so wide that any agreement reached by Tuesday would likely be vague and wouldn’t necessarily be written down in a formal text, U.K. Foreign Secretary Philip Hammond warned Friday…

U.S. and European officials said it is unclear if such a political statement will be enough to keep critics of the deal on Capitol Hill at bay.

Senior U.S. officials also have privately warned that White House might not have the political support in Washington to extend the negotiations—without at least a broad political agreement announced with Tehran by Tuesday.