Vienna, Associated Press—Talks over Iran’s nuclear program are making little headway, with Tehran resisting US-led efforts to crimp activities that could be turned toward making weapons, diplomats said Monday.

As negotiations move closer to a July 20 target date for a deal, both sides are trying to plug holes in a sketchy draft agreement.

Five days into the latest round of talks between Iran and six global powers, two diplomats told The Associated Press that there is still a disagreement on the constraints Iran is ready to accept in exchange for a full end to sanctions stifling its economy. The diplomats demanded anonymity because they are not authorized to discuss the confidential negotiations.

The biggest hurdle remains uranium enrichment, a process that can make reactor fuel or the core of a nuclear weapon, depending on the grade of material produced. Iran, which insists it does not want such arms, now has nearly 20,000 centrifuges either on standby or churning out reactor-grade fuel.

Tehran has long demanded that it be allowed to run up to 50,000 centrifuges to power its one existing nuclear reactor, and the two diplomats said Monday’s expert talks began with no formal change in that position.

The United States wants no more than a small fraction of that number. Its strongest backers at the negotiating table are Britain, France and Germany, with Russia and China leaning towards agreeing on any deal acceptable to Tehran and Washington.

The diplomats said there’s still disagreement over how to minimize proliferation dangers from a nearly built reactor that would produce substantial amounts of plutonium—like enriched uranium, a potential pathway to nuclear arms.

In addition, Iran is resisting pressure to convert an enrichment site dug into a mountain as protection against air attack to another use, they said. Differences also exist over the length of any agreement placing limits on Tehran’s nuclear activities.