The amount of money which South Africa's MTN Group is alleged to have illegally moved out of Nigeria is "mind boggling", a Nigerian lawmaker said on Thursday at the start of a parliamentary investigation into the telecoms company.

He did not specify what the alleged amount might be.

Nigeria's upper house of parliament agreed last month to investigate whether the company unlawfully repatriated $13.92 billion between 2006 and 2016.

MTN shares fell 3.2 percent to more than six-year lows after Bloomberg quoted a senator as saying the amount could be "outrageously higher". The shares later pared some losses.

The accusations of illegal money transfers were denied by MTN Nigeria Chief Executive Ferdi Moolman, who appeared at the hearing.

"MTN categorically denies all suggestions that its bankers in strict violation of the FEMM Act (Foreign Exchange (Monitoring and Miscellaneous Act) and Foreign Exchange Manual, repatriated $13.92 billion illegally out of Nigeria," said Moolman.

The allegation is the latest setback for Africa's biggest telecoms firm in its most lucrative but increasingly most problematic market, coming months after it agreed to pay a greatly reduced fine of 330 billion naira ($1.08 billion) to end a long-running dispute over unregistered SIM cards.

MTN officials, Nigeria's trade minister and four lenders were due to appear at the parliamentary hearing, which started on Thursday.

"It is not what we can sweep under the carpet. We intend toget to the bottom of the fact, all of the fact, nothing but the fact," said Olusola Adeyeye, a senator, reading out a statement from Senate leader Bukola Saraki, said at the start of hearing.

Dino Melaye, the senator who first made the allegations in a motion passed by lawmakers last month, said he raised the issue "as a patriotic Nigerian, a whistle blower and an anti-corruption crusader", adding that he stood by the allegations.

MTN is the largest mobile network operator in Nigeria, Africa's most populous nation and has the continent's biggest economy. The country accounts for a third of MTN's revenue.It follows a row between South African hotel and gaming group Sun International and Nigeria's Economic and Financial Crimes Commission over its investment in a hotel operator in Lagos. The disagreement caused Sun International to pull out of Nigeria in August.

Rafiu Ibrahim, chairman of Nigeria's senate investigative panel on alleged illegal repatriation of funds, said on Wednesday that a team of international and local accountancy experts and lawyers had been assembled to look into the matter.

"Investors are nervous with what's happening with Nigeria purely because the past history has shown us that typically the government tends to move the goal posts," Independent Securities trader Ryan Woods said.

($1 = 304.5000 naira)