Sri Lanka said Friday it would protest to the United States over its allegations that army shelling killed hundreds of families during the final days of the island's ethnic civil war.
A senior foreign ministry official said the allegation, made in a US embassy tweet Thursday, would be discussed with visiting Ambassador for Global Criminal Justice Stephen Rapp.
"Our minister (G. L. Peiris) will take it up face-to-face with ambassador Rapp," the official told AFP, asking not to be named.
"It is a baseless allegation. It is uncalled for," he said.
Rapp arrived in Sri Lanka Monday on a five-day visit to meet officials and politicians to discuss Sri Lanka's rights record and attempts at reconciliation five years after the end of war.
The US embassy posted on Twitter a photo of Rapp and its ambassador Michele Sison from the island's former battle zone with the caption: "St Anthany's Ground - site of Jan 2009 killing of hundreds of families by army shelling."
US diplomats in Colombo told AFP the tweet reflected Washington's toughening human rights policy towards Sri Lanka.
"This tweet reflects official government stance," an embassy official said, speaking on condition of anonymity.
Sri Lanka has denied charges that up to 40,000 ethnic Tamil civilians were killed by the army during the push that crushed Tamil rebels fighting for a separate homeland.
In other tweets, the embassy said Rapp and Sison also visited a pro-opposition newspaper in Jaffna, the capital of the battle-scarred Northern Province and expressed "concern for media freedom in Sri Lanka."
The two went to the Uthayan newspaper and met its owner, Eswarapatham Saravanapavan, who is an opposition legislator from the Tamil National Alliance party.
Saravanapavan said his paper had been a target of attack and five of his employees have been killed in recent years, but no suspects have been prosecuted.
The Sri Lankan foreign ministry official said Colombo believed the US charges were aimed at laying the ground for renewed condemnation of Sri Lanka at the March UN Human Rights Council (UNHRC) sessions in Geneva.
"The statement could be seen as a justification what they are planning," the official said.
Immediately after the US tweets, Sri Lanka's military added it feared a wider campaign by Washington against Colombo.
"This could be part of a much larger campaign that they (the US) intend on launching in the near future," Sri Lanka's military spokesman Brigadier Ruwan Wanigasooriya said in a statement.
Sri Lanka has come under mounting pressure to probe allegations its troops committed war crimes during the decades-long conflict.
In March, the UNHRC is due to discuss two previous US-initiated censure motions against Colombo over alleged violations of international humanitarian law.
Britain's Prime Minister David Cameron warned during a Commonwealth summit in Colombo two months ago that he would push for an international probe into alleged Sri Lankan human rights violations unless President Mahinda Rajapakse's government investigated the charges.
Sri Lanka maintains not a single civilian was killed by troops and in November ordered a census of war casualties.
The country's main Tamil political party has rejected the count as a sham and says it will collate its own figures.
The UN estimates the conflict for a separate homeland for ethnic Tamils in the Sinhalese-majority nation cost at least 100,000 lives.