The outline of the U.S. offer to Iran in their high-stakes negotiations over Tehran’s nuclear program is starting to become clearer.

Iran is currently reviewing a formal U.S. nuclear deal proposal as both sides remain deeply divided over uranium enrichment limits and sanctions relief.

A senior Iranian official told CNN the new nuclear deal proposal presented to Tehran in recent days is “incoherent and disjointed,” as sources familiar with the progress of the talks said the momentum behind negotiations to secure a new deal appears to be collapsing.