Diplomats following nuclear talks between Iran and six world powers say two days of negotiations have failed to narrow differences standing in the way of agreement by a July 20 target date.

Two diplomats spoke to the Associated Press on Thursday ahead of a third day of planned meetings, with the possibility that they would wrap up the round early because of the lack of progress.

One Western diplomat told news agency AFP that Iran’s position in the talks was “worrying.”

“It is worrying that there is no evolution on the part of the Iranians on most subjects, including sanctions,” the diplomat said on condition of anonymity.

Differences between the two sides on uranium enrichment, the central issue not only in this fifth round of talks but for the past decade, remain “major,” the envoy said.

“We have drafted little bits (of an accord) but the complex issues haven’t really been addressed in the drafting process,” he added.

The key dispute centers on uranium enrichment, which can produce both reactor fuel and the fissile core of a warhead. The US and its allies want deep cuts in Iran’s enrichment program. Tehran, which says it does not want nuclear arms, is resisting.

They aim to get a deal by the time an interim accord agreed in November expires on July 20, although the deadline can be pushed back by up to six months if both sides agree to do so.

Last month’s round ended in deadlock.