Around 14 camps were affected in the northwestern province of Idlib, David Swanson of the UN Office for the Coordination of Humanitarian Affairs told AFP.
The Idlib region, controlled by Syria's former al-Qaeda affiliate, is home to more than 3 million people - more than half of them displaced by the country's eight-year war.
Civil defence workers known as the White Helmets have been working to save peopleand their scant belongings from the rising muddy waters.
"For the second day in a row, White Helmets... continue to respond to the catastrophic situation in the northern Syria camps," they said on Twitter late on Monday.
One video posted by the group on Sunday showed brown water cascading out of a flooded tent.
In another published the same day, civil defence workers clung on to a rope as they waded through a brown torrent above knee level.
The downpour has affected tens of thousands of civilians, displaced persons, crops and livestock in Idlib, as well as in the Aleppo and Hasakeh provinces since Saturday, Swanson said.
In Aleppo province, tents were destroyed in several camps for the displaced and a hospital in the countryside had to shut down due to the flooding.
Syria's war has killed more than 370,000 people and displaced millions since starting in 2011 with the brutal repression of anti-government protests.
Tens of thousands of displaced Syrians in the north of the country depend on handouts from humanitarian aid groups, including food, blankets and heating fuel for the winter months.
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