Politically sensitive talks aimed at ending a decades-old dispute over Iran’s nuclear programme are now in their final stages.
The writing of a final agreement is expected to begin next week, according to Mohamad Javad Zarif, Tehran’s Secretary of Foreign Affairs.
It comes after an “historic” preliminary accord was struck in Lausanne, Switzerland, after marathon talks between Iran and the US, Britain, France, Russia and Germany.
But sharply differing interpretations emerged on what the framework agreement covers – a sign of what diplomats and nuclear experts say will be tough talks ahead.
The aim is to block an Iranian path to a nuclear bomb, in return for lifting sanctions on Tehran.
The deadline for a final deal is 30 June.
Here are some of the key issues causing debate:
The lifting of sanctions
The lifting of crippling sanctions imposed against Iran is at stake as part of any nuclear deal.
Tehran wants the sanctions, which include nuclear-related United Nations resolutions as well as US and EU nuclear-related economic sanctions, to be lifted at once.
President Hassan Rouhani has said that he will not accept a comprehensive deal unless this happens.
But America says that sanctions against Iran would be removed gradually under any agreement.
In what is seen as a setback for Barack Obama, the US president has agreed that Congress should have the power to review any deal with Iran – backing down to pressure from Republicans and some in his own party.
The move blocks Obama’s ability to waive many US sanctions on Tehran, while Congress reviews the deal.
It also allows Congress a final vote on whether to lift sanctions imposed by US lawmakers.
Washington, as well as negotiators from Iran and other members of the six-power group, has for months voiced concern that Congress could fatally undermine a final deal.
The prospect of sanctions relief being delayed or rejected altogether would add to the concerns in Iran.
Beefing up international monitoring of Iran’s nuclear work could become the biggest stumbling block to a final accord.
As part of the preliminary agreement, Tehran and the world powers agreed that United Nations inspectors would have “enhanced” access to remaining nuclear activity in Iran, where they already monitor key sites.
But details on exactly what kind of access the inspectors will have have been left for the final stages of the talks, posing a major challenge for negotiators on a complex and logistically challenging issue that is highly delicate for Iran’s leaders.
Iran says its nuclear programme is peaceful, but it has never welcomed intrusive inspections and has in the past kept some nuclear sites secret.
A delegation from the International Atomic Energy Agency (IAEA), the United Nations watchdog, recently arrived in Tehran for scheduled technical talks.
Talks with the IAEA are parallel to Iran’s nuclear negotiations with the powers seeking the permanent agreement.
Aside from the question of Iranian consent, the logistical requirements for increased monitoring of Iranian nuclear sites would be daunting.
It would involve more cameras, on-site inspections, satellite surveillance and other methods.
It might require the IAEA to assign more people and resources to its Iran team.
Israel has been hugely critical of the preliminary accord struck in Lausanne. It claims that the agreement will not prevent Tehran from developing nuclear weapons.
Israeli Intelligence Minister Yuval Steinitz has said that his country is happy with the Congress deal struck in the US.