Embattled Nigerian Senate president Bukola Saraki on Tuesday brushed off allegations of wrongdoing concerning his wife's offshore assets revealed in the Panama Papers as he defended himself in court Tuesday against fraud charges.

he latest graft claim to hit the senate president emerged from the "Panama Papers" investigation into a trove of 11.5 million tax documents leaked from Panamanian law firm Mossack Fonseca, which specialises in creating offshore shell companies.

Saraki is alleged to have failed to declare at least four offshore assets listed under his wife Toyin's name that appear in the leaked documents, according to investigation media partner Nigerian newspaper Premium Times. Under Nigerian law, it is mandatory for the president, the vice-president, state governors and their deputies to declare their assets along with those of their wife and children under 18 when they take office and before stepping down.

But Saraki said he did not do anything illegal and argued that the assets are listed as part of his wife's "family estate." "I've fully complied with law on asset declaration," Saraki said in a statement issued on Monday posted on his website.

"The law does not require a public officer to declare assets held by the spouse's family," Saraki's spokesman Yusuph Olaniyonu said. "It is public knowledge that Mrs. Saraki comes from a family of independent means and wealth with numerous and varied assets acquired over decades in family estates and investments."

Saraki is standing trial in Abuja, Nigeria's capital, before the Code of Conduct Tribunal facing charges including false declaration of assets while he was governor of the western state of Kwara from 2003 to 2011. He has denied all charges. A trained physician and former banker, the senate president is considered Nigeria's third most senior politician behind President Muhammadu Buhari and Vice-President Yemi Osinbajo.

Yet anti-corruption campaigners fear that the powerful politician will, like others before him, outmanoeuvre the law. "The latest revelation about Saraki's family should not surprise anybody," Debo Adeniran, chairman of the Coalition Against Corrupt Leaders lobby group, told AFP.

"We suggest that the Nigerian anti-graft agencies should collaborate with their foreign partners to move against Saraki and make him accountable," Adeniran said. "If Saraki escapes the Nigerian laws because of the loopholes and leniency in our laws, the international community should not allow him to escape.

"He should get the Ibori's treatment," Adeniran said, referring to the case of former Delta state governor James Ibori who was acquited in Nigeria on corruption charges but jailed in London for a similar offence. Several high-profile politicians are currently standing trial as part of Buhari's drive to tackle endemic corruption in Nigeria, Africa's largest crude producer and biggest economy.