US intelligence officials have expressed their concern over the still small but slowly-increasing presence of American citizens among Islamist rebel groups in Syria, according to a report by The Daily Beast.

Out of the estimated 1,200 rebel factions battling the Syrian regime of Bashar al-Assad, only a handful are either formally linked with Al Qaeda – such as the Al Nusra Front and the Islamic State of Iraq and al-Shams (ISIS) – or inspired by its ideology without having formal links to the group.

Nevertheless, such groups are increasingly gaining influence over other more “moderate” factions, and are the source of the majority of foreign fighters present in Syria, many of whom already identify with the Al Qaeda ideology.

According to the report, while hundreds of European nationals are reportedly fighting alongside rebel forces, a far smaller number of foreign fighters in Syria – anywhere between 10-60 – are believed to be US citizens.

However, American intelligence agencies are still concerned that those who survive the Syrian battlefield could return to the US, battle-hardened and motivated by an extreme, anti-western ideology – posing a real danger to American security.

“At some point all of these people from Europe are going home. All the folks there from all over the world, including the United States, will be coming home if they do not meet their end on the battlefield,” said Rep. Mike Rogers, the Republican chairman of the House Permanent Select Committee on Intelligence, on Tuesday.

Online chatter between Islamist extremists indicated that Syria was fast becoming a “new Afghanistan,” with extremists viewing it as fertile territory for their activities.

This is not the first time that concerns have been raised about the presence of western-born and raised Al Qaeda fighters in Syria, including on the front lines of key battles.

In July, a US passport was found in an abandoned ISIS base, after Kurdish fighters pushed back Islamist forces in northern Syria following fierce clashes.

Last month, a rebel faction comprised of foreign fighters was said to have led a successful assault on a strategically-important airbase in the flashpoint city of Aleppo.