Human Rights Watch on Tuesday called on the International Criminal Court to investigate possible "war crimes" in Sudan's Darfur region where fighting has intensified despite calls for an end to the conflict.

The New York-based group charged that the Rapid Support Forces (RSF) and allied Arab militias "summarily executed" at least 28 members of the Massalit ethnic minority when they ransacked and torched much of the town of Misterei in May.

Efforts to broker an end to the violence have continued, and the east African regional bloc IGAD on Monday led a renewed push, calling on the warring parties to "sign an unconditional ceasefire".

The Sudanese army nonetheless boycotted the gathering in Addis Ababa, dampening hopes for an end to the nearly three-month-old conflict with the RSF.

Experts believe army leader Abdel Fattah al-Burhan and his former deputy, RSF commander Mohamed Hamdan Daglo, have opted for a war of attrition and are hoping to extract more concessions at the negotiating table.

US ambassador John Godfrey, who along with other diplomats was evacuated near the start of the conflict, warned that "a military 'victory' by either of the belligerents in the Sudan conflict would entail unacceptable human cost and damage to the country".

- 'Negotiated exit' -

Godfrey called instead for "a negotiated exit from the crisis", which he said "does not -- and cannot -- mean returning to the status quo that existed before April 15".

Before the conflict erupted that day, the two generals had jointly ruled the country following an October putsch that derailed Sudan's fragile transition to civilian rule.

Around 3,000 people have been killed in the violence, according to the Armed Conflict Location and Event Data Project.

Godfrey further slammed "irresponsible" calls for continued fighting, pointing to the "horrific deaths by air strike of at least six people" in Khartoum North on Monday and of "at least 22 people on Friday in Omdurman".

US Assistant Secretary of State for African Affairs Molly Phee was due to meet regional and Sudanese officials Tuesday in Addis Ababa.

IGAD on Monday said it would request the African Union to look into possibly deploying the East Africa Standby Force -- which is usually tasked with election observer missions -- in Sudan "for the protection of civilians and... humanitarian access".

But such a move would likely face hurdles as the conflict has seen multiple successive ceasefires violated, preventing humanitarian aid from reaching the places most in need.

Kenyan President William Ruto, who leads the IGAD quartet tasked with finding a solution to the Sudan conflict, on Monday reiterated calls for humanitarian passages.

But Sudan's foreign ministry has repeatedly objected to his leadership of the quartet, accusing Nairobi of siding with the RSF.

- Executed in schools -

The RSF paramilitaries have meanwhile been accused of a litany of abuses, including some that the United Nations said could amount to crimes against humanity.

Human Rights Watch on Tuesday said dozens were killed and wounded when "several thousand" RSF fighters attack in late May on Misterei in Darfur.

Arriving at dawn "on motorcycles, horses or pickup trucks", the fighters launched an attack that resulted in the "near total burning of the town" of 46,000 residents, the watchdog said.

In response, armed members of the Massalit -- one of the key non-Arab communities in Darfur -- clashed with the RSF.

The RSF "and allied Arab militias summarily executed at least 28 ethnic Massalit", HRW said.

Civilians were "executed" in schools and mosques where they had sought shelter, witnesses told HRW.

The group charged that the violations "amount to war crimes" and called on the ICC to investigate the attack on Misterei.

The Hague is already investigating war crimes committed in Darfur during the conflict that erupted in 2003 and saw the Janjaweed -- the precursor to the RSF -- unleashed on ethnic minority rebels there in a conflict that killed over 300,000 people.