The United States and United Nations have said the situation in Sudan’s region of West Darfur could herald a repeat of past mass atrocities there as fighting in Sudan reached its third month.

The war between Sudan’s army and the paramilitary Rapid Support Forces broke out on April 15 but quickly spread westward, hitting cities in the Kordofan and Darfur regions.

Activists said El Geneina, on the border with Chad, has been particularly badly hit. Fighting has killed 1,100 people and sent more 270,000 refugees across the border to Chad. Homes and hospitals have been destroyed.

The United States blamed the Rapid Support Forces and allied militias fo the violence. But army aircraft and drone attacks had impeded humanitarian efforts, it said.

The situation there was “an ominous reminder of the horrific events that led the United States to determine in 2004 that genocide had been committed in Darfur,” the U.S. State Department said in a statement.

In the early 2000s, Sudan’s army relied on Arab militias to put down a rebellion by armed Darfuri groups. Those militias, known as the Janjaweed, formed the origin of the Rapid Support Forces, which evolved into a force that was legalized in 2017.

Then-president Omar al-Bashir and aides are wanted by the International Criminal Court for genocide and crimes against humanity after 300,000 people were killed and millions displaced.

“Darfur is rapidly spiraling into a humanitarian calamity. The world cannot allow this to happen. Not again,” U.N. aid chief Martin Griffiths said in a statement on Thursday.

A letter from several U.S. and Sudanese activist and civil society groups called on RSF commanders to be held accountable for failing to rein in their soldiers and for the army to be held accountable for not protecting civilians.

The U.S. statement said army aircraft and drone attacks had impeded humanitarian efforts.

The U.N. refugee agency said it had heard reports of “shocking incidents of sexual violence,” including by fighters entering civilian homes and stopping them at checkpoints as they tried to flee conflict zones.

Human trafficking, particularly in East Sudan, was on the rise, it said.