Speaking to the BBC Sandeshaya during his visit to the UK, President Sirisena said an investigative commission will be appointed legally and it will be tasked with identifying what needs to be done to probe the alleged violations.
If the commission found anyone guilty of committing war crimes they will be punished according to the country's law, Mr. Sirisena said.
The President however stressed that there is no need for the UN war crime investigators to get involved in the commission's investigations.
"We have clearly asked them to trust our domestic mechanism. We will consider their advice and strengthen our domestic inquiry," Mr. Sirisena told BBC's Saroj Pathirana.
"I don't think we need any one coming from outside for the investigations, but their advice will clearly be of better use for our activities," he said.
The President said he expects the investigative committee tasked with examining alleged human rights abuses would work "efficiently, in a balanced, legal and impartial manner".
When questioned about the LTTE suspects still detained without being charged, the President said he has instructed the National Security Council to provide him with a report on whether they can be charged or released. He added that the detainees that would be released based on the report will have the same opportunity as the other citizens of the country to lead a normal life.
He insisted there was now space for open dialogue and dissent on issues including war crimes. But he said he "doesn't believe" war crimes allegations contained, for instance, in the documentary "No Fire Zone", which was shown on the UK's Channel 4.
Speaking of the discussions he had with the British Prime Minister David Cameron on Tuesday, the President said Mr. Cameron has a good understanding of Sri Lanka's conflict, post-conflict period and the present situation. He said the discussion with the British PM was extremely fruitful.
The President stressed that he was committed to reconciliation and said his government expects 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.
