A U.N. human rights expert urged the next U.N. secretary-general on Friday to make the elimination of tax havens a priority to ensure that corporations, billionaires and "kleptocrats" pay their fair share of taxes.

Alfred de Zayas, an American law professor, also urged Antonio Guterres, who will succeed Ban Ki-moon as U.N. chief on Jan. 1, to call a world conference on phasing out the offshore havens.

He said the United Nations "must no longer tolerate the scandal of secrecy jurisdictions that facilitate tax evasion, corruption and money-laundering."

De Zayas told a news conference after presenting his new report to the U.N. General Assembly that it's estimated that as much as $32 trillion are held offshore in secrecy jurisdictions escaping just taxation.

He said governments lose $3 trillion every year in tax evasion and tax avoidance schemes "and most perpetrators have enjoyed immunity."

De Zayas, who is the U.N. expert on the promotion of a democratic and equitable international order, said the key issue is transparency.

"Once you have transparency, the tax havens are useless, but the problem is the secrecy jurisdictions," he said.

De Zayas' report cited The Tax Justice Network Financial Secrecy Index, which ranks jurisdictions according to their secrecy and the scale of their offshore financial activities.

The top three jurisdictions listed in 2015 were Switzerland, Hong Kong and the United States.

Other "high-profile jurisdictions" listed include the United Kingdom and its territories and dependencies, Luxembourg, the Netherlands, Belgium, Malta, Cyprus, Singapore, Liberia and Panama.

De Zayas urged the General Assembly to draft a convention to outlaw tax havens worldwide.

"The United Nations must take concerted action against abuses and crimes perpetrated by individuals, speculators, hedge funds and transnational enterprises who skirt taxes and loot governments," he said.