Forbes Israel has released a list of the world's wealthiest terror networks and placed the Iraqi Sunni insurgent group Islamic State (Isis) on top.

According to the report, the extremist organisations use money-making methods ranging from drug trafficking to robberies and extortion to donations via NGOs in order to finance their resources.

A notable amount of money has been transferred to terror groups by NGOs, charities, financial institutions and even government organisations, said the financial group.

The IS, which is currently terrorising Iraq and Syria seizing key oil fields and refineries to boost their capabilities, has an estimated net worth of $2 billion.

According to the US Treasury's recent estimates, the Iraqi group manages to earn about $1 million a day by selling crude oil but according to Forbes, it is making nearly three times the figure.

The recently-formed Iraqi group is followed by the Palestine-based Hamas, which has an approximate annual turnover of about $1billion.

Hamas was able to enter the "big league" due to its takeover of Gaza in 2007 which allowed the group to tax businesses and civilians alike in the region.

The al-Qaeda Islamist organisation along with all its global affiliates stands at sixth place just behind the Revolutionary Armed Forces of Colombia, the Lebanon-based Hezbollah and the Taliban.