The Ethiopian military and its allies are responsible for widespread sexual violence against women in Tigray, using rape as a strategy of war, Amnesty International says.

The scale of violations during the nine-month conflict in the north of the country amounts to war crimes, the human rights group says.

One woman reported being gang-raped in front of her children.

Ethiopia's government says the report is based on "flawed methodology".

The foreign ministry accused Amnesty of waging "sensationalised attacks and smear campaigns" against Ethiopia, saying the country condemned sexual violence in all circumstances.

Amnesty said it had "overwhelming evidence" to show such violence had been rampant since the start of the conflict.

It began last November when the region's Tigray People's Liberation Front (TPLF) party stormed a military base after falling out with the prime minister over his political reforms.

Warning: Some people may find details in this story upsetting.

The rights group interviewed 63 women and children from Tigray who said they had been raped by Ethiopian troops, their Eritrean allies, or by pro-government fighters from the neighbouring region of Amhara.

The author, Donatella Rovera, told the BBC that the testimonies had been among the worst she had ever heard.

"The level of sadistic and gratuitous brutality in addition to the rape was absolutely shocking," she said.

A 39-year-old woman reported being seized by Eritrean soldiers as she was travelling with her two children. "Five of them raped me in front of my children," she told Amnesty.

"They used an iron rod, which is used to clean the gun, to burn me. They inserted pieces of metal in my womb... Then they left me on the street."

Some women Amnesty interviewed said they had been detained for weeks and repeatedly raped, often by several men.

Almost half of the women accused Eritrean soldiers of carrying out the violations, identified by their Tigrinya accents and uniforms.

The Eritrean authorities did not respond to Amnesty's requests for comment.

Amnesty is calling on the UN to send a team of experts to Tigray to further investigate the allegations that may amount to crimes against humanity.

The widespread nature of the assaults suggests military officials knew what was happening and that it was being tolerated at the highest level of government in Ethiopia and Eritrea, Amnesty said.

"The patterns of sexual violence emerging from survivors' accounts indicate that the violations have been part of a strategy to terrorise, degrade, and humiliate both the victims and their ethnic group," the report said.

Ms Rovera said the victims of the assaults have not had access to the support they need to recover and many are living in poverty in Sudanese refugee camps, or camps in Tigray.

On Tuesday, when Ethiopian Prime Minister Abiy Ahmed called on civilians to join the army to fight in Tigray, he also accused Tigrayan forces, made of up the TPLF and its allies, of recruiting child soldiers, raping women and blocking aid.

Amnesty says none of the women it had interviewed accused Tigrayan forces of rape, but said that it would be monitoring the situation now the conflict had spread beyond Tigray's borders.

The TPLF, which has been designated a terrorist organisation by the government but says it is the legitimate regional government of Tigray, has accused the federal authorities of blocking aid.