Fighters could endanger their home countries

The number of Islamic State fighters is swelling as more recruits leave their home countries to join the militants, posing a tougher challenge for the United States in its fight against the extremist group.

The IS has overtaken vast swaths of territory in Syria and northern Iraq in recent months, and a UN report released earlier this month found that the ranks of groups like al-Qaida and IS may have increased by more than 70 percent over the last nine months, with fighters coming from about 100 different countries.

There are currently about 25,000 foreign fighters attached to the IS group, according to the report.