GENEVA — The United Nations human rights chief, Zeid Ra’ad al-Hussein, sharply rebuked the Sri Lankan government on Friday for “unacceptable conduct” for what he said were efforts to obstruct an investigation into possible war crimes in Sri Lanka.

The “continuing campaign of distortion and disinformation” as well as “insidious” attempts by the authorities to intimidate witnesses were an affront to the United Nations human rights body that had ordered the investigation, Mr. Zeid said.

His strongly worded statement was the latest twist in a diplomatic struggle to conduct a credible investigation of events in 2009, at the end of Sri Lanka’s brutal civil war with Tamil Tiger separatists. An earlier panel of experts appointed by the United Nations secretary general, Ban Ki-moon, concluded that up to 40,000 civilians were killed, mainly as a result of deliberate artillery bombardment by the army.

The United Nations Human Rights Council in Geneva ordered the investigation in March after a series of inconclusive inquiries by the Sri Lankan authorities and in the face of fierce diplomatic resistance by the government to an external examination.