The International Criminal Court is investigating a surge in hostilities in Sudan's Darfur region since mid-April, including reports of killings, rapes and crimes affecting children, the top prosecutor told the United Nations on Thursday.

The regular army and paramilitary Rapid Support Forces (RSF) have been battling in the capital Khartoum and other areas of Sudan in a power struggle that exploded in mid-April.

More than 3-million people have been uprooted, including more than 700 000 who have fled into neighbouring countries.

UN Secretary-General Antonio Guterres said last week that Sudan, Africa's third largest country by land area, was on the brink of full-scale civil war that could destabilise the wider region.

"The office can confirm that it has commenced investigations in relation to incidents occurring in the context of the present hostilities," the International Criminal Court (ICC) Prosecutor Karim Khan's office said in a report to the UN Security Council.

ICC prosecutors are "closely tracking reports of extrajudicial killings, burning of homes and markets, and looting, in Al Geneina, West Darfur, as well as the killing and displacement of civilians in North Darfur and other locations across Darfur," the report said.

It is also examining "allegations of sexual and gender-based crimes, including mass rapes and alleged reports of violence against and affecting children," it said.

In El Geneina, witnesses have reported waves of attacks by Arab militias and the RSF against the non-Arab Masalit people, the largest community in the city, that have sent tens of thousands of people fleeing to nearby Chad.

While the ICC cannot currently work in Sudan due to the security situation, it intends to do so as soon as possible, the report said. Under a 2005 UN Security Council resolution, its jurisdiction is limited to the Darfur region.

The ICC has four outstanding arrest warrants related to the earlier fighting in Darfur between 2003 and 2008, including one against former Sudanese President Omar al Bashir on charges of genocide.

Al Bashir and two of his former ministers who are also wanted by the ICC for alleged war crimes in Darfur had been in custody in Sudan. The army said Bashir and one of the former ministers, Abdelrahim Mohamed Hussein, had been moved to a military hospital before the outbreak of the fighting. The other former minister, Ahmed Haroun, said he had broken out of prison with others 10 days after the start of the conflict.

Khan said he had sent a request to Sudan's government, which has a long history of not cooperating with the ICC, to find out the current location of the suspects.

In April, the ICC opened its first trial dealing with Darfur crimes in the case of alleged Janjaweed leader Ali Muhammad Ali Abd-Al-Rahman.

While applauding the ICC's investigation, the US State Department on Thursday condemned continued "atrocities and ethnically targeted killings" committed in West Darfur.

"The atrocities and violence in Darfur demand accountability, meaningful justice for victims and the affected communities, and an end to impunity," department spokesperson Matthew Miller said in a statement.