Secretary-General Ban Ki-moon says 34 militant groups from around the world had reportedly pledged allegiance to the Islamic State extremist group by mid-December — and the number will grow in 2016.

The U.N. chief called IS "the world's wealthiest terrorist organization," saying numerous reports estimate it generated $400-$500 million from oil and oil products in 2015.

Ban said in a report to the U.N. Security Council on Friday that taxing businesses, farmers and other economic activity, and confiscating and reselling homes have become another significant source of revenue over the past year.

He said IS poses "an unprecedented threat" because it can quickly adapt, including to declines in income, and because of its ability to persuade groups from the Philippines to Uzbekistan, Pakistan and Libya and Nigeria to pledge allegiance.