The number of foreign fighters joining the Islamic State group has doubled since last year to as many as 31,000 people, a report by the New York based intelligence consultancy Soufan Group said Tuesday. The finding seemed to indicate that efforts to prevent foreign fighters from reaching the militant stronghold in Syria and Iraq have so far had a limited effect.

"The foreign fighter phenomenon in Iraq and Syria is truly global," the report said, according to the Agence France-Presse. "The Islamic State has seen success beyond the dreams of other terrorist groups that now appear conventional and even old-fashioned, such as Al-Qaeda."

A similar study by the group in June 2014 found that about 12,000 foreigners were fighting alongside the militant group, but the recent study found between 27,000 and 31,000 foreign fighters had joined. They represented some 86 countries. A majority of the fighters traveled to ISIS-held territory from the Middle East and North Africa – about 8,000 foreign fighters from each region. Around 5,000 fighters have come from Europe, and another 4,700 from former Soviet republics, the AFP reported.

An estimated 20 percent to 30 percent of foreign fighters return home, according to the report. Returning fighters have been a source of concern for many countries, particularly in the aftermath of the coordinated attacks last month in Paris, which left at least 130 people dead and hundreds more wounded. Some of the attackers had previously traveled to Syria. In the aftermath of the attacks, national security experts have urged a new emphasis on de-radicalization programs to reintegrate individuals into society.

ISIS has recently experienced significant setbacks. Amid an anti-ISIS U.S.-led airstrike campaign, as well as a separate Russian campaign, ISIS has lost control of considerable territory. The group has also suffered financial strain due to the targeting of its oil infrastructure. The militants continue to bring in about $80 million per month, according to a recent study, but have been forced to cut fighters’ salaries, increase the cost of electricity and impose new taxes.

"Even if the Islamic State is a failing enterprise in steady decline, it will be able to influence the actions of its adherents, and it may become more dangerous as it dies," the Soufan Group report said.