34 militant groups have pledged allegiance to ISIS: Ban Ki-moon

He further added that UN member states should also prepare for an increase attacks by ISIS associated groups

Thirty-four militant groups from around the world had reportedly pledged allegiance to the Islamic State extremist group as of mid-December and that number will only grow in 2016, UN Secretary-General Ban Ki-moon said in a report.

Ban said UN member states should also prepare for an increase attacks by ISIS associated groups from countries such as the Philippines, Uzbekistan, Pakistan, Libya and Nigeria, traveling to nations.

"The recent expansion of the ISIL sphere of influence across west and north Africa, the Middle East and south and southeast Asia demonstrates the speed and scale at which the gravity of the threat has evolved in just 18 months," Ban said, using another abbreviation for the group.

Adding to the threat, ISIS is "the world's wealthiest terrorist organization," Ban said, citing estimates the group generated $400-$500 million from oil and oil products in 2015, despite an embargo.

According to the UN mission in Iraq, cash taken from bank branches located in provinces under ISIS control totaled $1 billion. The mission also estimates that a tax on trucks entering IS controlled-territory generates nearly $1 billion a year, he said.

The extremist group captured large swathes of Iraq and Syria less than two years ago and despite international efforts to oust them, Ban said IS continues to maintain its presence in both countries and is expanding to other regions.