Ranil Wickremesinghe said talks were under way to establish a credible domestic mechanism to investigate abuses during the decades-long conflict with Tamil separatist rebels that ended in May 2009.
"There is nothing to be got from abroad," Wickremesinghe said, after a damning UN report recommended Colombo allow international experts to assist its domestic investigation.
"The media says hybrid (inquiry), but it is not hybrid," said Wickremesinghe, after UN rights chief Zeid Ra'ad Al Hussein asked the government to establish "a hybrid special court, integrating international judges, prosecutors, lawyers and investigators".
Members of Sri Lanka's Tamil minority say they do not trust a local inquiry to reach the truth about the conflict, in which more than 100,000 people died.
It ended in 2009 with the defeat of rebels who had waged a long fight for a separate homeland for the Tamils.
Read: UN calls for special court to prosecute Sri Lanka war crimes
President Maithripala Sirisena's new government has vowed to punish war criminals, in contrast to his hawkish predecessor Mahinda Rajapakse who had insisted that not a single civilian was killed by troops under his command.
Sri Lanka became an international pariah after repeatedly resisting calls for a credible probe into the horrendous crimes, including the killing of at least 40,000 Tamil civilians in the final months of the war.
But in a major shift Washington last month announced it would support Colombo's plans for a domestic inquiry, which is also supported by neighbouring India.
When Sirisena came to power in January - backed strongly by the Tamils - he promised to restore human rights and the rule of law as well as mend fences with regional power India and the West.
His government has proposed a series of new measures to promote reconciliation and accountability after accusing the previous administration of breaking promises to deliver justice.
Wickremesinghe said he expected the United States to move a resolution at the ongoing UN rights council sessions backing his administration.
"Discussions are going on in Geneva so I don't want to talk about it, but we hope the US will bring a consensus resolution on Sri Lanka," he said.
The new government announced last week that it would set up a South African-style truth commission, a war reparations office and a commission on missing people.
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