Suspected US-led coalition air strikes killed at least 26 civilians in a Syrian village, piling pressure on the alliance after allegations another bombing raid left regime soldiers dead.
The coalition has been bombarding the Islamic State group for more than a year in Syria and neighbouring Iraq, where the jihadists have declared a self-styled caliphate.
But according to a monitoring group, strikes on Monday on the village of Al-Khan in northeastern Syria only left civilians dead.
Rami Abdel Rahman of the Syrian Observatory for Human Rights said IS is in control of Al-Khan but is only on its outskirts, "which is why all of the deaths were civilians".
The death toll included at least seven children and four women, he said, adding that it was likely to rise as more than a dozen civilians were still missing under rubble.
A spokesman for the US-led coalition said he had no details yet about the raid, but that a "credibility assessment" would review claims of civilian deaths.
Last month, the US said four civilians were "likely" to have been killed in strikes against IS in Iraq. And in November 2014, it admitted accidentally killing two children in a strike in Syria.
The Al-Khan strike came with the coalition already under pressure over allegations it carried out a raid the previous day that killed Syrian soldiers, in the first such case.
In a letter to the UN Security Council and Secretary General Ban Ki-moon, Syria accused the coalition of targeting an army camp in the eastern province of Deir Ezzor on Sunday, killing three soldiers and wounding 13.
The foreign ministry letter condemned the attack as "a flagrant aggression".
The Observatory said four soldiers died in the first incident of US-led strikes killing Syrian troops.
A Syrian military source gave the same toll, and said the attack late Sunday hit several buildings used as weapons depots and an army training camp, damaging two tanks.
But a coalition spokesman said its only strikes in the area on Sunday were on an oil wellhead some 55 kilometres (35 miles) southeast of the army base, and a US official, speaking on condition of anonymity, blamed Russian strikes for the deaths.
Much of Deir Ezzor is under IS control, but the regime still has a presence in small areas, including in the provincial capital.
The province's oil has been a major source of IS funding, but on Monday analysis firm IHS said the group was suffering financially due to air strikes on its oil infrastructure.
IHS estimated IS's overall monthly income to be about $80 million (75 million euros) as of late 2015, around half of it from levies and confiscations.
But it noted the group also had significant costs because it administers large swathes of territory.
'Lacks seriousness and credibility'
The Syrian government has regularly criticised the US-led strikes as ineffective and illegal because they are not coordinated with regime forces.
Civilians inspect a shattered building in Syria's northern city of Aleppo on December 7, 2015 following a reported air strike by Syrian government forces.
"The US coalition lacks the seriousness and credibility to effectively combat terrorism," the foreign ministry said.
Staunch regime ally Moscow began its own aerial campaign in Syria on September 30 and coordinates its strikes with Damascus.
On Sunday, US President Barack Obama vowed to destroy IS and hunt down its followers at home and abroad.
It followed a shooting rampage in California last week that saw an apparently radicalised couple kill 14 people.
While pledging to "hunt down terrorist plotters in any country", Obama also said he would not be "drawn once more into a long and costly ground war in Iraq and Syria".
"They know they can't defeat us on the battlefield... but they also know that if we occupy foreign lands, they can maintain insurgencies for years, killing thousands of our troops and draining our resources, and using our presence to draw new recruits," he said.
Elsewhere, Syrian media said four people were killed in rebel rocket fire near the now-closed Russian consulate in Aleppo city.
Also on Monday, the powerful Jaish al-Islam group announced it will attend a major meeting of Syrian opposition forces in Riyadh this week, the first rebel group to confirm participation.
The group is the most prominent armed opposition faction near Damascus and controls most of the Eastern Ghouta suburb, which is regularly bombarded by government forces.