The United States on Friday urged Sri Lanka to heed calls to investigate claims that government troops killed 40,000 civilians in a 2009 assault on the Tamil Tiger rebels.
"They should implement the plan that they have never implemented and they should listen to the words of the international community," State Department spokeswoman Victoria Nuland told reporters.
She welcomed a resolution passed by the UN Human Rights Council on Thursday which had pressed Colombo to "credibly investigate widespread allegations of extrajudicial killings and enforced disappearances."
The US sponsored the move with the support of 24 other nations after months of frustration that Sri Lanka was failing to investigate the accusations of human rights abuses in the offensive which ended the three-decade civil war.
Sri Lankans were warned when they visited Washington "that if they didn't take steps... they would see this kind of a resolution coming forward. That was about six months ago, and we didn't see any progress," Nuland said.
If Sri Lankan authorities still continued to ignore the calls for a full investigation, then the United States would look at what further measures could be taken, she added.
The UN estimates that some 40,000 people were killed in the final months of Sri Lanka's civil war, mostly in indiscriminate shelling by government forces during a final onslaught against Tamil separatists.
Sri Lanka insists that no civilians were killed by its troops