Often driven by poverty, some "2,000 to 4,000 have in the past three years turned their back on their secular states to seek a radical alternative," the International Crisis Group said in a briefing on the region.
That figure is much higher than the official one of several hundred given by the governments of Uzbekistan, Tajikistan, Kazakhstan, Turkmenistan and Kyrgyzstan, the report said.
It said that ethnic Uzbeks were the largest group from Central Asia fighting with the Islamic State, but "Kyrgyz, Kazakhs, Turkmen and Tajiks are also well represented."
IS has previously released footage purporting to show adult and child recruits from Central Asian states.
An IS propaganda video that came out in November alleged to show young children from Kazakhstan training to become fighters for the jihadi extremists.
One of the boys -- who identified himself as Abdullah from Kazakhstan -- then appeared in another video this month that apparently showed him executing two alleged Russian spies with a pistol.
"Dozens" of fighters from Central Asia are thought to have died in the ranks of IS and their families are sometimes only informed via text messages or on social networking sites, the Crisis Group said.
In December, Uzbek President Islam Karimov expressed his "utmost concern" at the "expansion of militant extremism", while Tajik President Emomali Rakhmon last month said "the Islamic State is the plague of the new century".
The report warned, however, that secular governments in the region are ill-prepared to deal with the problem, either in preventing radicalisation or coping with the return of fighters.
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People from the region are drawn to join Islamic State by poverty and lack of opportunities, but there is no one single profile, the report said, with those recruited including young women.
Officials with Soviet backgrounds have a "limited understanding of religion’s appeal in society", the report said.
The think-tank urged the United States and European countries to "recognise that Central Asia is a growing source of foreign fighters and consider prioritising policing reform."
Western countries should promote "a more tolerant attitude to religion, in their recommendations for combating the problem," it added.
Known collectively as "Chechens" within Islamic State, the Central Asians are often ill-prepared for fighting, the Brussels-based group reported, citing an aid worker calling them "human material".
Returning fighters can expect the harshest possible treatment from Uzbekistan and Kazakhstan, it said, with a Russian official being quoted as saying that "Uzbekistan will shoot them at the border".
Central Asian governments "are using the threat to bolster political agendas while curtailing civil liberties, but they have yet to create a credible counter-action plan," it warned.