The possibility of holding a sixth round of talks between the US and Iran on a nuclear deal is in serious doubt and may not happen at all, a source said, CNN reports.

A senior Iranian official on Monday denounced the new US proposal on Iran's nuclear program, saying that at “at first glance, is assessed as incoherent and disjointed, very unrealistic, and with excessive demands.”

“The fact that the Americans constantly change their positions has so far been the main obstacle to the success of the talks and now makes the work more difficult than ever,” the official added.

Also, the official alleged the latest text directly contradicts prior understandings.

“The text is clearly in conflict with the latest agreement reached during the fifth round of negotiations,” the official stated.