The US embassy's official Twitter handle uploaded a photo with a caption saying, "St Anthany's Ground-site of Jan 2009 killing of hundreds of families by (Lankan) army shelling".
Sri Lanka had for long dismissed accusations that army had fired shells at civilian locations.
In May 2009, Sri Lankan military had defeated the Tamil Tiger rebels. Colombo has resisted calls to probe claims that over 40,000 ethnic minority Tamils were killed by the military during the final phase of the civil war.
Stephen J Rapp, US Ambassador-at-Large at the Office of Global Criminal Justice, is on a two day visit to Jaffna - the area that saw the worst of the nearly three-decade-long war - as part of his week long visit to the country.
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The embassy's move is seen with interest by observers ahead of the 3rd US sponsored resolution on Sri Lanka to be moved at the UN Human Rights Council in March. Two previous resolutions were adopted with India's support. Both called for expeditious action to achieve reconciliation with the Tamil minority.
A protest was sparked when Rapp told the main Tamil party TNA that the March resolution would push for a international war crimes investigation.