Islamic State (IS) has released a video appearing to show the beheading of a Kurdish man as a warning to forces fighting the group in northern Iraq.
The video, entitled a Message in Blood, shows several men in orange jumpsuits said to be captured Kurdish fighters.
The victim is then seen kneeling near a mosque in the IS-held city of Mosul before he is beheaded.
The jihadists warn that others will be killed if Kurdish leaders continue to back the US.
Kurdish "peshmerga" fighters from the autonomous Kurdistan Region of northern Iraq have been trying to counter an advance by IS, backed by US air strikes.
Earlier, IS videos from Syria appeared to show the mass killing of Syrian soldiers taken prisoner after their base was overrun.
Activist group the Syrian Observatory for Human Rights said the soldiers had been captured while trying to flee to Hama province after IS stormed the Tabqa airbase.
The Sunni militant group has declared a "caliphate" across the parts of Syria and Iraq that it controls.
Last week, IS drew worldwide condemnation after releasing a video showing the beheading of US journalist James Foley. It said his death was in retaliation for US air strikes in Iraq, and the jihadist group threatened to kill other US hostages.
Analysis by BBC Arab Affairs Editor Sebastian Usher
Entitled A Message in Blood, the latest video produced by the skilled propagandists of Islamic State is clearly aimed at the Kurds fighting them in northern Iraq.
It parades a group of Kurdish men who are said to be captured peshmerga fighters and who have all been put in orange jumpsuits. That detail is not the only way in which this video is reminiscent of the IS staging of their murder of the US journalist, James Foley.
The beheading of one of the Kurdish hostages is shown filmed from two angles - both to add extra documentary proof and to make it a slicker production. But the backdrop this time is not made as neutral as possible. Instead, the main mosque in Mosul looms behind - a provocation, trying to show that IS control of the city is assured.
The screen is then split showing the hostage and his captors on one side and photos of Kurdish leaders meeting President Obama and his Secretary of State John Kerry on the other. The jihadists threaten more such killings if the Kurds continue to ally themselves with the US. In a choreography of murder again very similar to the killing of James Foley, the kneeling man is then beheaded with a knife.
On Thursday, US President Barack Obama said he was sending Secretary of State John Kerry to the Middle East soon to discuss the Syria crisis with regional partners, especially those who adhere to the Sunni faith.
"I'm encouraged so far that countries in the region, countries that don't always agree on many things, increasingly recognise the primacy of the threat that Isil (IS) poses to all of them. And I've asked Secretary Kerry to travel to the region to continue to build the coalition that's needed to meet this threat," he said.
Mr Obama was speaking shortly before convening a meeting of his national security advisers on a range of Pentagon options for confronting IS.
Earlier on Thursday, a video posted online purportedly showed the aftermath of the killings of the Syrian soldiers - a long line of bodies of young men lying face down stripped to their underwear.
"The 250 Shabiha taken captive by the Islamic State from Tabqa in Raqqa have been executed," read the caption, using the name - Shabiha - given by the opposition to militiamen fighting for President Bashar al-Assad.
An earlier video appears to show the same men, also in their underwear, being marched through the desert at gunpoint. They are made to walk and run, with those at the back being kicked and beaten.
Militants shout "Islamic State" and "There is no going back".
Tabqa airbase - near the northern city of Raqqa, an IS stronghold - fell to IS on Sunday after weeks of fierce fighting.
The Observatory said 346 IS fighters and more than 170 members of the security forces were killed in the final battle, which lasted five days.
The head of the Observatory, Rami Abdul Rahman, told the AFP news agency that about 1,400 troops had been stationed at the airbase, 700 of whom managed to escape.
On Tuesday, UN investigators issued a report which said IS militants had committed "mass atrocities" in Syria and had recruited children as fighters.
The report said public killings were a "common spectacle" in areas run by the jihadist group and that local people were forced to watch.
At the same time, the investigators said that Syrian government forces had also committed atrocities by dropping barrel bombs and chlorine gas from helicopters, shelling hospitals and torturing and killing civilians.