The axial U_A(1) though an anomalous symmetry is believed to affect the order of the chiral phase transition in QCD with two light quark flavours. In this talk I discuss about our study of the axial anomaly in finite temperature QCD using first principles lattice gauge theory technique. We use chiral overlap fermions to probe the underlying topology of dynamical QCD configurations with two light quark flavours generated with Highly improved staggered quarks. From the eigenvalue spectrum of the overlap operator we find no evidence of effective restoration of U_A(1) near the chiral transition temperature $T_c$. A pile up of the near-zero eigenmodes is observed to persist even at $1.5 T_c$ which is primarily responsible for its breaking . These eigenmodes are localized unlike those in the bulk, with a mobility edge similar to a Mott–Anderson like system. We find evidence in support of a dilute gas of instantons at the highest temperature studied, as the microscopic origin​ ​of the breaking of U_A(1) symmetry.