The United States has warned it will not trade concessions on Iran's nuclear program for Tehran's co-operation in combating the Islamic State group.
Ahead of the UN General Assembly in New York, and with nuclear talks with Iran, known as the P5+1, at a critical point, Washington made its most explicit statement yet that the two issues were not linked - reflecting growing political pressure on the White House.
"The United States will not be in a position of trading aspects of Iran's nuclear program to secure commitments to take on ISIL," White House spokesman Josh Earnest said.
"These two issues are entirely separate, and the focus of the P5+1 talks will remain on resolving the international community's concerns about the Iranian nuclear program."
Iran's supreme leader Ayatollah Ali Khamenei said last week that his government had rejected a request from the US for co-operation on the battlefield.
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US officials have not confirmed or denied making an offer in private, but they do not regard Iran as part of the coalition they are building to fight the IS group.
They have also been adamant that Iran could not expect a watering down of the US negotiating stance in nuclear talks in return for Tehran's help in battling IS, which has carved out a haven in vast tracts of Syria and Iraq.
Israeli Prime Minister Benjamin Netanyahu warned on Sunday that the West should not ease sanctions on Iran to win its support in the fight against the jihadists.
