The UN Human Rights Council passed a resolution Thursday urging Sri Lanka to "credibly investigate" allegations of war crimes during its battle against Tamil Tiger separatists in 2009.
Unlike in the immediate aftermath of the conflict when it staved off a similar move at the Human Rights Council, Colombo was unable this time to lobby sufficient support to defeat the Western-led move, with even its ally India voting in favour.
Tabling the resolution, the United States said Colombo had been given three years to hold its own investigations into allegations of serious violations, but "given the lack of action… it is appropriate" that the 47-member state council pushed it to do so.
"An enduring peace will be unsustainable without meaningful steps to foster national reconciliation and accountability," said US envoy Eileen Donahoe.
"It is a resolution that encourages Sri Lanka to … make concerted efforts at achieving the kind of meaningful accountability upon which lasting reconciliation efforts can be built."
The US envoy also noted that India’s backing was "very helpful because they are such a close neighbour".
"We see India’s support as nothing but positive."
Rights groups say up to 40,000 civilians died in the final months of Colombo’s military campaign to crush the Tamil Tigers, who waged a bloody decades-long campaign for a separate homeland for minority Tamils.
The UN estimates some 100,000 people died during Sri Lanka’s ethnic conflict between 1972 and 2009.
Sri Lankan authorities however angrily rejected the resolution, with ruling party MPs even rallying outside parliament in Colombo while the bill was being considered in Geneva.
Sri Lanka’s human rights envoy Mahinda Samarasinghe told the council it was a "misconceived, unwarranted and ill-timed resolution which embodies several harmful elements that clearly violate important principles that will have adverse ramifications, not only for my country, but many other countries."
He insisted that Sri Lanka must be given time to allow its domestic investigations to run its course, and warned that the resolution would be counter-productive.
It would "also undermine the principle of non-interference in matters within the domestic jurisdiction of a country," he added, a point backed by China and Russia, both of which rejected the resolution.
Some activists also complained of "intimidation" tactics by Colombo-backed elements, with people who appeared to be with the Sri Lanka delegation going around photographing certain members of NGOs.
"It was done in a very obvious manner and we felt uncomfortable," an activist told AFP on condition of anonymity, adding that the team "contemplated leaving the session".
The group brought a complaint to the presidency of the UN Human Rights Council, and later decided to stay as an investigation was launched into the matter.
The complaints and investigation was confirmed by a UN rights spokesman who however said he could not give further details.