Three of Biden's team negotiating with Iran on nuclear deal RESIGN because the US is not being tough enough - as diplomats warn talks are entering a critical phase

• Senior Iran negotiator quits amid reports of policy differences
• State Department official confirmed Tuesday that Richard Nephew stood down
• He was  U.S. Deputy Special Envoy for Iran and known as sanctions architect
• Nephew, who wanted Biden to take a harder stance against Iran, has reportedly been avoiding the meetings in Vienna since December 
• At the same time, reports emerged that two other negotiators had left
• It comes at a critical time in negotiations between the West and Tehran 
• Iran has rejected talk of an interim agreement and wants a legal guarantee that the U.S. will not walk away from the nuclear deal
• It also won't negotiate directly with the US, with European intermediaries