He was speaking to the BBC's Sinhala service while on a visit to London.
The Sri Lankan army defeated the Tamil Tiger separatists in 2009, ending a 26-year long conflict. Both sides have been accused of atrocities.
President Sirisena said he expected the investigative committee tasked with examining alleged human rights abuses would work "efficiently, in a balanced, legal and impartial manner".
He said he was committed to reconciliation: "We expect to begin a new journey to promote reconciliation, cohabitation, brotherhood and friendship among the people of Sri Lanka, and to win over international opinion on these issues."
