In recent months much of the focus has been on Syria's chemical weapons and the prospects of peace talks but with shifting alliances and infighting among opposition factions the situation is now more complex than ever.
Last week, the United States and the UK decided to suspend some non-lethal aid to opposition fighters in northern Syria because of worries it may fall into the hands of extremist groups after Islamist fighters recently took over bases and warehouses from the Free Syrian Army (FSA).
The majority of the fighters in Syria are Syrians ... the foreign fighters ... have to leave the country, because we don't need fighters, just we need weapons and ammunition to be able to resist the forces of the regime and to collapse the regime.
With apparent defections on all sides this leaves the FSA increasingly splintered. Its influence on the ground was always questionable and now it appears to have hardly any sway at all over the myriad of armed factions.
Efforts by the head of the Syrian National Coalition, Ahmed Jarba, to restructure opposition fighters into a national army were never welcomed by many inside Syria who described it as a western project.
The EU says it is also becoming increasingly concerned about Europeans going to fight in Syria and joining groups such as al-Qaeda.
There are reports saying that the number of rebel fighters from 11 western European countries has nearly doubled in the past six months.
The Belgium government estimates up to 2,000 are from the European Union while Paris says more than 180 fighters are from France. Britain's Mi5 estimates that fighters from the UK range in the hundreds. There are also reports estimating that 500 people from the Balkans, including Bosnia and Kosovo, are fighting there.
"In the beginning in Syria you had very few radical groups, then al-Qaeda came from Iraq and now you have territories that are really dominated and controlled by al-Qaeda, its own territory, right at the doorstep of Turkey, so not too far from Europe. And they of course have fighters who come from North Africa and the European Union. Obviously this creates a relatively serious security problem because we have to anticipate their return, and how to handle this," Joelle Milquet, the Belgian interior minister, says.
Some analysts believe the threat is not as serious as the EU is making out. But as the war spills over into neighbouring countries like Lebanon, the rise of foreign fighters is often seen as a dangerous development; and it is likely to only make matters worse.
Inside Syria, with presenter David Foster, discusses with General Salim Idriss, the chief of staff of the rebel umbrella group, the Free Syrian Army.
"I hope that they [ the foreign fighters] have the same [as] what we have in mind - to collapse the regime and to build a free and democratic Syria, but I am afraid that many of them have other goals and that's why we like to say and to tell them 'We don't need foreigner fighters..."
General Salim Idriss, Free Syrian Army