Negotiators at the six-country talks in Vienna on Iran's nuclear programme forged a hard-won preliminary agreement in April and now must agree on the details of the deal by the end of June.
The deadline is under threat because of the potential for late-term meddling with the terms by opponents in Iran and the United States.
U.S. President Barack Obama, under pressure, signed legislation that gives the U.S. Congress a say in the terms. Meanwhile Iran's Supreme Leader, Ayatollah Ali Khamenei asserts that Iran won't agree to "unreasonable demands," which include denying access to Iranian nuclear scientists by international inspectors.
On Apr 2 Iran agreed to scale back its nuclear programme significantly for 10 to 15 years and to accept stringent international inspections in exchange for the lifting of the banking and energy sanctions that have crippled the Iranian economy.
The remaining sticking points include the details of the inspections, the number of centrifuges Iran will be allowed to keep and how rapidly the sanctions will be lifted.
Tehran asserts that its nuclear program is aimed at producing nuclear energy, not nuclear weapons.
The six countries - the United States, United Kingdom, Russia, China, France and Germany -- have been in negotiations with Iran to finalize the preliminary deal reached in Geneva in 2013.
Date written/update: 2015-05-22