Turkey and Azerbaijan deny that Syrian mercenaries were used in the recent offensive in Nagorno-Karabakh but four Syrians have told the BBC that after enlisting for sentry duties in Azerbaijan, they were unexpectedly thrown into battle on the front line.

BBC's political analyst Ed Butler reports that it was back in August of this year that the rumours started to circulate in rebel-held areas of northern Syria: there was well-paid work to be had overseas.

"I had a friend who told me that there is a very good job you can do, just to be at military checkpoints in Azerbaijan," one man told me.

"They told us our mission would be to serve as sentries on the border - as peacekeepers. They were offering $2,000 a month! It felt like a fortune for us," said another, whom I will call Qutaiba.

But the work wasn't what it seemed. The men, many of them with no military experience, were being recruited for war - as they soon discovered when they were taken to the front line and ordered to fight.

"I didn't expect to survive," Qutaiba says. "It seemed like a 1% chance. Death was all around us."

Estimates vary as to the exact death toll among the Syrian fighters. The Syrian Observatory for Human Rights, a UK-based group that monitors the conflict in Syria, puts the figure at more than 500. This compares to reported figures of more than 2,400 on the Armenian side, and nearly 3,000 on Azerbaijan's side, though Azerbaijan doesn't acknowledge that any Syrians were among them.

"We don't use mercenaries," the country's president, Ilham Aliyev, told the France 24 news network in October.

Military analyst Michael Kofman, head of the Russia programme at the CNA military research centre in Washington DC, says the goal seems to have been to minimise Azeri troop casualties.