Fighters of the Islamic State stand guard at a checkpoint in the Northern Iraq City of Mosul
Fighters of the Islamic State of Iraq and the Levant (ISIL) stand guard at a checkpoint in the northern Iraq city of Mosul, June 11, 2014. Picture taken June 11, 2014. REUTERS/Robert Stringer

A monitoring group has revealed that ISIS has murdered more than 150 captured soldiers in Syria in the last two days. According to several reports, video images posted online showed Syrian soldiers being walked through the desert with nothing but their underwear. Some images contained men lying dead in the sand. The mass killings of soldiers appeared to mark ISIS' victory to gain control of Raqqa Province's Tabqa airbase.

A gruesome video of the slaughter was posted on YouTube showing the gory turn of events. Reports said the video has not been independently verified. The footage began with dozens of men believed to be Syrian soldiers being herded in the desert like cattle in their underwear. The video then fades to back to an image showing a stack of men's bodies red with blood.

According to reports, the exact death toll is not clear since other sources said it was 250 while others suggested lower than that number. However, majority had agreed that at least 150 bodies were visible in the video.

In the video's description on YouTube, it said the footage contained the execution of Syrian army officers and Nusayri people. The Nusayris are a significant minority group of Shia Muslims in Syria, reports said.

Another version of the video had the caption that said "250 shabeeha" were captured by ISIS from Tabqa in Raqqa and executed. Shabeeha means soldiers loyal to Syrian President Bashar al-Assad, according to Islamists.

The New York Times reported that a Lebanese who joined ISIS and identified only as Yousef said the men who were executed in the video were part of the military forces. The men tried to rescue troops who left the air base. The men were "ambushed" and arrested, Yousef said.

According to the Syrian Observatory for Human Rights, an opposition group monitoring deaths in Syria, more than 150 soldiers were captured and killed by ISIS since Aug. 27. However, the organisation's director, Rami Abdulrahman doubted ISIS' accounts. He said he has seen nothing in the video to prove that the executed men in the video were the same soldiers who were sent to the area.

The Syrian Foreign Ministry has released a statement and warned that the government will hold terrorist groups and forces supporting them responsible for the safety of captured soldiers. The Syrian government has demanded militants to release them immediately.

WATCH VIDEO HERE. Warning: Graphic Content