PHOTO: Iranian Foreign Minister Mohammad Javad Zarif and the European Union’s Catherine Ashton meet on Tuesday night in Vienna

Iran and the 5+1 Powers (US, Britain, Germany, France, China, and Russia) open their fourth round of talks on a comprehensive nuclear agreement today in Vienna.

The two sides are seeking a final resolution by July 20, when an interim Joint Plan of Agreement expires.

Leading issues include the number and development of Iran’s centrifuges for enriching uranium. Tehran has 19,000 centrifuges, of which 9,000 are operational, and has said it wants to add thousands more.

The US and European countries say Iran needs far fewer than 19,000 centrifuges for a civilian nuclear program. Although Iran has suspended production of 20% uranium — a necessary step to any enrichment beyond 90% for a nuclear warhead — Washington and its allies fear the retention of the 19,000 centrifuges could give Iran a “break-out capability” to move toward militarized use within several weeks.

One solution may be to set out Iran’s program in “work units”, which would also offer a better measure of uranium production taking into account the status of Tehran’s older and newer models of centrifuges.

The status of Iran’s nuclear facilities will also be discussed. There appears to be some movement towards agreement on the under-construction Arak heavy-water nuclear reactor, which has concerned Western powers because of plutonium by-product that can be used in a military program.

Iranian officials are offering a re-design of the reactor to reduce the plutonium by 80% and have also indicated that its on-line date could be delayed beyond the end of 2014.

Tehran is seeking the lifting of all sanctions, especially those which have crippled its energy and banking sectors since 2011.

Any US and European steps are being linked to Iran’s compromise on the centrifuge and nuclear facilities issues, as well as Tehran’s agreement to a rigorous system of inspections and supervision.

Since November, Iran and the International Atomic Energy Agency have reached a series of agreements leading towards that system. However, the US and European allies may also seek measures overseeing “possible military dimensions” of Iran’s nuclear program.

Tehran has rejected any formal measure mandating inspection of military bases and has said that consideration of its missiles is not on the agenda of the talks.