Islamic State (IS) militants have been accused of massacring hundreds of people in areas under their control in northern Iraq and eastern Syria.
At least 80 members of the Yazidi religious minority are believed to have been killed with women and children abducted in a village in Iraq.
IS is also accused of killing 700 tribesmen opposing them in Syria's Deir Ezzor province, over a two-week period.
The violence has displaced an estimated 1.2 million people in Iraq alone.
US aircraft are providing air support for Kurdish forces in northern Iraq to drive back the militants and retake the strategic Mosul dam.
US Central Command said it carried out nine air strikes on Saturday, hitting targets near Irbil and the dam.
"All aircraft exited the strike areas safely."
The UK, Germany and other countries are delivering humanitarian aid for refugees in the north.
IS first emerged in Syria, fighting President Bashar al-Assad during the ongoing civil war there, but it has since overrun parts of northern Iraq, making its capital in the city of Mosul.
Pursuing an extreme form of Sunni Islam, it has persecuted non-Muslims and Shia Muslims, whom it regards as heretics.
'MEN AND BOYS'
Kurdish and Yazidi sources say the attack on the Yazidis took place in the village of Kawju (also spelt Kocho), near the town of Sinjar, on Friday afternoon.
Reports say the men were killed after refusing to convert to Islam. A US drone strike later destroyed two vehicles belonging to the militants.
Men were separated from women and children under 12 years old and the men and male teenagers were led away in groups of a few dozen each and shot on the edge of the village, a wounded man who escaped by feigning death told AP news agency.
The fighters then walked among the bodies, using pistols to finish off anyone who appeared to still be alive, the 42-year-old man said by phone from an area where he was hiding, on condition of anonymity.
"They thought we were dead, and when they went away, we ran away," he said. "We hid in a valley until sundown, and then we fled to the mountains."
A Yazidi refugee from a different village, Moujamma Jazira, told AFP news agency that people there had also been massacred, after trying in vain to fight back.
Dakhil Atto Solo said that 300 men had been executed in his village, and the women and children abducted. The report could not be verified independently.
Meanwhile, UK-based Syrian opposition activists reported that IS had executed 700 members of the al-Sheitaat tribe in the oil-rich province of Deir Ezzor.
"Reliable sources" reported that many of the tribesmen had been beheaded, the Syrian Observatory for Human Rights said.
"Those who were executed are all al-Sheitaat," Observatory director Rami Abdelrahman told Reuters news agency by telephone. "Some were arrested, judged and killed."
The news comes after reports on social media in recent days of al-Sheitaat tribesmen being decapitated by militants, their severed heads left in public view in streets.
Tribesmen in the area had tried to drive out IS at the beginning of the month, in a rare display of local resistance. IS responded by rushing in reinforcements.
RALLIES IN EUROPE
Kurdish forces supported by US air strikes battled on Saturday to retake Mosul dam, the country's largest, from IS.
Kurdish Peshmerga fighters shelled militants' positions and there was an unconfirmed report of a ground attack.
In north-eastern Syria, Kurdish forces have been giving military training to Iraqi Yazidis to help them fight IS, Reuters news agency reports.
In Western cities, demonstrators marched in support of Iraq's minorities, Yazidis, Christians and others, on Saturday.
In Paris, Kurdish protesters rallied against the "genocide of Yazidi Kurds" while in Hannover, Germany, a protester at a march told AP news agency: "Politicians must act now against the genocide... where not only Yazidis but also Christians, Arameans, Assyrians, heretics and anyone who doesn't agree with the inhuman world view of the terrorists of the IS is massacred, even decapitated. Now is the time for politicians to act."
Two Airbus flights carrying UK aid supplies landed in the Kurdish city of Irbil on Saturday and German military transport planes have also begun delivering aid.
German Foreign Minister Frank-Walter Steinmeier said on a visit to Baghdad that he hoped the new Prime Minister, Haider al-Abadi, would be able to represent all the different regions and religions in the country, as this was the only way to prevent disenchanted Iraqis from backing IS.