"Because of the intensity of international pressure, there is a disincentive to engage in earnest in a domestic process," Xinhua quoted Peiris as saying at a conference held in Colombo.
"And that is why this international pressure is not only not helpful, but is absolutely harmful."
In March, the UN Human Rights Council (UNHRC) passed a resolution led by the US and Britain to investigate the last seven years of Sri Lanka's 26-year conflict between the government and the Liberation Tigers of Tamil Eelam (LTTE).
UN Human Rights Commissioner Navi Pillay last month announced the names of three experts who have agreed to advise and support the team set up to conduct a comprehensive investigation into alleged human rights violations in Sri Lanka.
The experts will play a supportive and advisory role, providing advice and guidance as well as independent verification throughout the investigation.
The Sri Lankan government has already rejected the proposed investigation and refused to allow the team members visas to enter the country to carry out the probe.
The government contends a domestic process will be the best path for reconciliation and has expanded the mandate of a presidential commission investigating people who went missing during the war to include allegations of civilian deaths caused by the Sri Lankan Army during the last phase of the conflict, which ended in May 2009.