Conflict in Iraq has led to nearly 15,000 civilian deaths and left 30,000 wounded during a 16-month period that ended on 30 April, according to a UN report.

The UN’s human rights office and its mission in Iraq said violations of international humanitarian law and gross human rights abuses by the Islamic State group, which controls large swaths of Iraq’s north and west, may in some cases amount to war crimes, crimes against humanity and possibly genocide.

Iraq is going through its worst crisis since the 2011 withdrawal of US troops. Isis captured Iraq’s second-largest city of Mosul and the majority of western Anbar province in 2014 and still holds large parts of the country, though Iraqi forces have made progress in recent months with the help of a US-led air campaign.

During the 16-month period, the report said, more than 2.8 million people fled their homes and they remained displaced in the country, including an estimated 1.3 million children.

The UN officers did not break down who was responsible for the casualties.

Though much of the report focuses on Isis, the UN agencies said they had received continued reports of violations of human rights and humanitarian law perpetrated by Iraqi security forces and their associates including international military forces, militia groups and popular mobilisation units. These include allegations of unlawful killings of people believed or perceived to support or be associated with Isis, particularly Sunni Arab community members, and several examples of reported civilian killings in air strikes.

The report gives numerous examples of killings, attacks and abductions carried out by Isis extremists against those opposed to its ideology, captured Iraqi soldiers and police, government officials, lawyers, journalists, doctors and other professionals, and members of ethnic and religious communities including Christians and Yazidis. It cites a number of unverified reports that Isis used or attempted to use chlorine gas in attacks.

As many as 3,000 to 3,500 men, women and children remained captives of Isis group, predominantly Yazidis but also members of other ethnic and religious communities, “where they are subjected to physical, sexual and other forms of violence and degrading treatment on a daily basis”, the report says.

Among the many killings cited in the report were that of a 47-year-old man publicly stoned to death on 31 March in Mosul for not following Isis’s instructions; 45 of its own members publicly executed in a square in central Mosul on 21 December for fleeing the battlefield; and two Sunni Arab men, aged 19 and 21, accused of homosexuality by one of the Isis’s self-appointed courts, who were thrown to their deaths from an eight-storey building in central Mosul in front of dozens of people on 16 January.