"The Sri Lankan government makes a lot of claims about pursuing accountability for wartime abuses, but the world is still waiting to see some results," said Brad Adams Asia director at Human Rights Watch.
"It's clearer than ever that an independent international investigation is needed to make genuine progress in providing justice for victims," HRW says in its 2014 World report.
The Sri Lankan government responded to a March resolution by the United Nations Human Rights Council by announcing various actions to provide accountability in accordance with its own Lessons Learnt and Reconciliation Commission.
HRW contends that Sri Lankans who criticised the government remained subject to harassment or threats.
"As 2014 begins, increased international pressure in support of basic rights is desperately needed to reverse a course that is threatening democratic rule in Sri Lanka," Adams said.
Sri Lanka has come under increasing international pressure to probe allegations of excessive civilian deaths during the final battle against the LTTE that ended in May 2009.