Around 75 fighters trained by American, British and Turkish military forces have entered Syria, a monitoring group has claimed.
The trained fighters crossed over from Turkey on Friday 18 and Saturday 19 September. They are now north of the Syrian city of Aleppo.
The fighters were trained and equipped by a Pentagon programme that US officials recently announced had fallen short of its aims.
A top US general told the senate that of the thousands of fighters the US military was supposed to train, only four or five remained in active service on the battlefield.
The Pentagon is expected to initiate a major overhaul of the programme, following a review of an initial group of 54 rebels that came under attack and suffered severe casualties.
Officials said the attack demonstrated that larger groups are necessary to stand a fighting chance against ISIS soldiers.
The fighters who have entered Syria have been supplied with four-wheel drive vehicles, mounted machine guns and ammunition. Warplanes from a US lead coalition are bombing ISIS in Syria and Iraq.
Russia also deployed fighter jets in Syria, shortly after holding talks with US officials about military action.
US Defence Secretary Ashton Carter spoke with his Russian counterpart last Friday, as Russian military involvement in Syria continues to escalate.
According to a statement released by the Pentagon, Carter and Russian Defence Secretary Sergei Shoygu had a “constructive conversation” where they agreed to “further discuss methods for deconfliction in Syria”.
The statement said Shoygu told Carter that recent Russian military activity is defensive, and he aims to support Russia’s obligations to the regime of Syrian President Bashar al-Assad.
US military officials have said they are wary of Russia’s aims with Syria, though they need to coordinate with leaders with aircrafts operating in the area.
Last Wednesday, Secretary of State John Kerry said he had spoken with Russian Foreign Minister Sergey Lavrov and was told that Russian movements in Syria were focused on the battle against ISIS. However, Kerry was sceptical about Russia’s intentions.
“I’m not taking that at face value, because we look at the type of airplanes or the type of munitions and so forth, and it obviously raises much more serious questions about what is happening,” he told reporters on Wednesday.
Kerry also said last Friday that the US government is committed to “finding a political settlement with respect to Syria, which cannot be achieved in the long-term presence of Assad”.