Iran is poised to reject a U.S. proposal to end a decades-old nuclear dispute, an Iranian diplomat said on Monday, dismissing it as a "non-starter" that fails to address Tehran's interests or soften Washington's stance on uranium enrichment.

"Iran is drafting a negative response to the U.S. proposal, which could be interpreted as a rejection of the U.S. offer," the senior diplomat, who is close to Iran's negotiating team, said.

The U.S. proposal for a new nuclear deal was presented to Iran on Saturday by Omani Foreign Minister Sayyid Badr Albusaidi, who was on a short visit to Tehran and has been mediating talks between Tehran and Washington.