The United Nations atomic agency expressed “serious concern” Friday about Iran’s failure to cooperate with its probe into undeclared nuclear material in the country, adding to calls from opponents of the 2015 nuclear deal with Tehran to kill off that agreement. In two reports sent to member states by the International Atomic Energy Agency, viewed by The Wall Street Journal, the Vienna-based organization said Iran had failed to give its inspectors access to two sites the agency wanted to visit. The agency also said Iran didn’t…