Between January and the end of July, 602,759 displaced Syrians returned to their homes, many of them citing an improved economic and security situation in the areas they had fled from, IOM said in a statement.
A total of 84 percent of those who have returned had taken refuge elsewhere within the war-ravaged country, while the remaining 16 percent returned from neighbouring countries Turkey, Lebanon, Jordan and Iraq.
More than a quarter of returnees said they did so to protect their assets and properties, while nearly the same number referred to the improved economic situation in their place of origin, and 11 percent cited the improved security situation there.
Fourteen percent meanwhile pointed to the worsening economic situation in their place of refuge, IOM said.
But while most of the returns had been spontaneous, it warned they were "not necessarily voluntary, safe or sustainable," it said.
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Aleppo Governorate, the scene of the harshest battles of Syria's bloody six-year war, had received 67 percent of all returnees so far this year, IOM said.
And within the governorate, Aleppo city, which was recaptured by the Syrian army last December after a suffocating five-month siege, has received most returnees, it said.
Many of those returning meanwhile must struggle to get bare necessities, with only 41 percent having access to clean water and 39 percent with access to health services.
This "is dangerously low as the country's infrastructure has been extremely damaged by the conflict," IOM said.
And even as returns from within Syria especially appear to be on the rise, the agency warned that the war-torn country is still seeing high rates of fresh displacement.
"From January to July 2017, an estimated 808,661 people were displaced, many for the second or third time, and over six million in total currently remain displaced within the country," it said.
More than 330,000 people have been killed in Syria since the conflict erupted in March 2011 with anti-government protests.