Iran is demanding that talks with the U.S. this week be held in Oman not Turkey, and that the scope be narrowed to two-way negotiations on nuclear issues only, a regional source said on Tuesday, adding new complications to an already delicate diplomatic effort, Reuters reported. 

Iran's move to change the venue and agenda for the talks, scheduled for Friday in Istanbul, came amid heightened tensions as the U.S. builds up forces in the Middle East.

Regional players have pushed for resolution of a standoff that has led to mutual threats of air strikes and stirred fears of escalation into a wider war, according to Reuters.