Liz Truss has said she hopes Britain will soon be in a position to pay the £400m debt overdue to Iran, according to an Iranian account of the phone call between the foreign secretary and her Tehran counterpart, Hossein Amir-Abdollahian. UK government officials have been exploring legal ways to pay Britain’s historical debt, although international economic sanctions on Iran have made it difficult. The UK Foreign Office did not proactively brief reporters on the call, but when contacted for comment it confirmed that the conversation had happened on Monday, saying: “We continue to explore options as a matter of urgency to resolve this case.” It said the UK had always been committed to paying the debt. With talks on the Iran nuclear deal reaching their decisive stage in Vienna, it is possible Truss wanted to stress the upside of both sides reaching a deal. The two countries insist that the £400m debt – dating back to the sale of tanks to the shah of Iran in the mid-1970s – has always been treated as independent of the Vienna talks and the fate of three British dual nationals held in Evin prison or, in the case of Nazanin Zaghari-Ratcliffe, denied the right to leave Iran. The tone of the call, according to the Iranian account, appeared to have been warmer than for some time, with Iran eager to see the UK do more to help with the Afghan refugee crisis on its borders. According to Iran, Truss praised Tehran for housing as many as 2 million refugees. The foreign ministry said: “Amir-Abdollahian positively assessed the relations between the two countries.” It may have been significant that the Irish foreign minister, Simon Coveney, was in Tehran on Monday. He has acted as a conciliator on political prisoner issues in the past. In the UK parliament last month, Boris Johnson again blamed the threat of sanctions for the block on the payment. The Vienna talks will mean many of sanctions could be lifted, but it has been argued that the UK has been responsible for its own sanctions laws since Brexit and so could pay the debt now if it is not tied up in a wider negotiation.