Russia's negotiator said Friday that there has been a "very worrying" slowdown in progress in nuclear talks between Iran and six major powers ahead of a June 30 deadline to finalise an historic accord.

"The rate of progress... is progressively slowing down," Sergei Ryabkov was quoted by RIA Novosti as saying as he arrived for the latest round of talks in Vienna. "This is very worrying to us because there is very little time before the deadline and we urgently need to enter the final stage."