VIENNA (REUTERS) - It will be very difficult to overcome differences between Iran and six world powers over Teheran's uranium enrichment programme, though all parties aim to adhere to their six-month deadline to reach a nuclear deal, a senior US official said on Wednesday.

"It's a gap (on enrichment) that's going to take some hard work to get to a place where we can find some agreement," the senior US administration official said after the latest round of negotiations on Iran's atomic programme in Vienna.

The official said the differences over Iran's planned Arak heavy-water reactor, which Western powers fear could yield weapons-grade plutonium, remained similarly wide.