Three Turkish soldiers have been killed and 10 wounded in an attack in Syria, but reports differ about what happened.
Turkey's military said the soldiers had been targeted in a Syrian government air strike on Thursday, during an offensive by Turkish-backed rebels against so-called Islamic State.
However, a Syrian monitoring group said the deaths were caused by a suicide bomb attack on Wednesday claimed by IS.
Meanwhile a member of the US military has died while fighting against IS.
The soldier, who has not been named, died from wounds caused by an improvised explosive device in Ayn Issa, north of the IS stronghold Raqqa.
Officials gave no other information. It is not clear whether this was linked to the deaths of the Turkish soldiers.
Syria's military has not yet commented on the fate of the Turkish soldiers.
It would be the first time Turkish soldiers have been killed by Syrian government forces in the offensive, which Damascus has denounced as a "flagrant violation of Syrian sovereignty".
Dubbed Operation Euphrates Shield, the offensive was launched three months ago with the aim of pushing IS militants away from the Turkish border.
The Turkish government also wants to contain US-backed Syrian Kurdish Popular Protection Units (YPG) militia, which it says is an extension of the outlawed Kurdistan Workers' Party (PKK) in Turkey.
So far, the rebels have driven IS militants out of more than 1,800 sq km (694 sq miles) of territory, according to the Turkish military, and recaptured the key border town of Jarablus and the symbolically important village of Dabiq.
They are now besieging the town of al-Bab, the last IS stronghold in Aleppo province.
The Turkish military statement said the soldiers who were killed were deployed in northern Syria when they were targeted at about 03:30 (00:30 GMT) in an air strike that it "assessed to have been carried out by Syrian regime forces".
It did not give an exact location, but the state-run Anadolu news agency reported that it was close to al-Bab.
The dead and injured soldiers, one of whom was said to be in a critical condition, were taken to hospitals in the southern Turkish provinces of Kilis and Gaziantep.
Later on Thursday, the UK-based Syrian Observatory for Human Rights disputed the Turkish military's account, saying the soldiers had been killed an IS suicide bombing on Wednesday near the village of Waqqah, north-west of al-Bab.
IS also issued a statement saying it had carried out a suicide bombing on a Turkish army position near Waqqah on Wednesday evening that caused an unspecified number of casualties.
Its self-styled news agency, Amaq, released video footage of the attack showing an explosives-filled armoured vehicle driving towards the frontline and then exploding.
The three deaths mean that at least 15 Turkish soldiers have been killed since Operation Euphrates Shield began, according to AFP news agency. Most died in clashes with IS, but one was killed in an attack blamed on the YPG.