Sri Lanka hopes to set up a domestic inquiry within a month into atrocities from the civil war, President Maithripala Sirisena has told the BBC.
He said UN investigators would not take part in the inquiry, but their views would be taken into account.
Mr Sirisena's predecessor Mahinda Rajapaksa consistently refused any co-operation with the UN.
The army and Tamil Tiger rebels were both accused of atrocities in the 26-year war, which ended in 2009.
Between 80,000 and 100,000 people are estimated to have died in the conflict.
In the final months, many thousands of Tamil civilians are thought to have been killed in government shelling.
Mr Sirisena told BBC Sinhala's Saroj Pathirana that an investigative committee would work "efficiently, in a balanced, legal and impartial manner".
When asked if UN investigators would be involved, the president said: "We are ready to get advice and their opinions for the inquiry, but I don't think we need any outsiders because we have all the sources for this."
Analysis: Charles Haviland, former BBC Colombo correspondent
Measured in his style, Maithripala Sirisena cuts a very different figure from his controversial predecessor Mahinda Rajapaksa.
On issues where Mr Rajapaksa always seemed implacable, President Sirisena appears to have opened the door just a little wider.
So Sri Lanka will still not allow UN war crimes investigators to visit. But, said the president, "we can take account of their opinion in doing our work, to make it more fruitful". That is worlds away from the old government's position.
His National Security Council will, he said, examine details of people, mostly Tamils, who have been in detention without charge, some for years, and will report back suggesting charging them or freeing them.
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.
The UN agreed last month to delay its long-awaited report into alleged war crimes, saying the new government was more willing to co-operate than the previous administration.
Mr Sirisena came to power in January, inflicting a surprise defeat on former ally Mr Rajapaksa.
Both men were in government during the bloody finale of the war, but fell out when Mr Sirisena defected.
Parliamentary elections take place later this year, and Mr Rajapaksa could return as prime minister.
Analysts say Mr Sirisena's refusal to allow UN investigators to be involved may be designed to appeal to Sinhalese voters, who are largely opposed to any international inquiry.
Mr Sirisena also received large numbers of votes from Tamils and Muslims in January's election victory.
As a result, he also has to promise to deal with the legacy of the war and promote national 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," he said.