The Syrian Observatory for Human Rights said on Tuesday that ISIS managed to recruit 6,000 militants in the past month alone.
The observatory’s director, Rami Abdel Rahman, said there were at least 20,000 non-Syrians among ISIS.
"July saw the largest recruitment since the group appeared in Syria in 2013, with more than 6,000 new fighters," said the head of the group, which opposes the Syrian government.
The observatory also said there were 1,000 foreign militants among the raw recruits mostly from Chechnya, Europe, Arab countries, and China.
Most of the militants had entered via Turkey, Abdel Rahman said, adding that other recruits joined ISIS from foreign-backed opposition groups, including 200 from the al-Qaeda-linked al-Nusra Front that initially cooperated with ISIS.
Syria has been gripped by deadly violence since 2011.
More than 170,000 people have been killed and millions displaced due to the violence.
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