These groups were often part of the plethora of Syrian rebel groups that operated in northern Syria since the Syrian civil war began in 2011.
Unregistered Syrians have been ordered to leave Istanbul, a move seen as politically motivated in light of Turkey's recent elections and growing discontent with the refugee population.
Syria gave its approval on Thursday for the UN to use Bab al-Hawa for another six months, but with several conditions.
Many opposition politicians and critical journalists have been jailed in Turkey, meaning politics is far from free.
Erdogan has said he aims to ensure the return of one million refugees within a year to the opposition-held areas.
The discontent in Turkey over the nearly 4 million Syrian refugees has been a source of domestic pressure on Turkish President Recep Tayyip Erdoğan.
Turkey has carried out several cross-border military incursions into northern Syria in recent years and has dozens of bases there.
The Turkish drone strike in Iraq shows the lengths Ankara will go to strike Kurdish anti-ISIS fighters.
In a tweet on Monday, he predicted "stronger seismic activity in the coming days," with a link to a YouTube video.
The number 6,600 likely means that other missing Syrians will also be added to this number, perhaps as many as 10,000 eventually.
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