GENEVA/ADDIS ABABA (Reuters) - All sides fighting in the war in Ethiopia's northern region of Tigray committed violations that may amount to war crimes, according to a long-awaited joint investigation by the United Nations and Ethiopia published on Wednesday.

       The report by the U.N.'s Office of the High Commissioner for Human Rights and the state-appointed Ethiopian Human Rights Commission was released the day after Ethiopia declared a state of emergency. Tigrayan forces said on Monday they might march on the capital to topple the government of Africa's second most populous nation.

       The report covers most of the year-long conflict, fought by Tigrayan forces against the Ethiopian military and its key allies: forces from Ethiopia's Amhara region and soldiers from the neighbouring nation of Eritrea.

       All sides are accused of torturing and killing civilians, carrying out gang-rapes and making arrests on the basis of ethnicity.

       It was not immediately clear whether findings from the report could form the basis for legal action. Ethiopia and Eritrea are not members of the International Criminal Court, so the court has no jurisdiction.

       The report recommended a possible international justice mechanism, saying Ethiopian investigations were insufficiently broad, did not always comply with international standards, and were not always transparent.

       (Additional reporting by Maggie Fick and Katharine Houreld; Editing by Angus MacSwan and Jon Boyle)