Kiev (CNN) US President Donald Trump's ex-campaign chair Paul Manafort is facing fresh allegations over his links to former pro-Russian Ukrainian President Viktor Yanukovych.

Ukrainian lawmaker Sergii Leshchenko claims a document found in a safe in Kiev may be evidence that Manafort tried to mask payments to him from Yanukovych's party.

The claim comes as the FBI told the House Intelligence Committee on Monday that it is officially investigating whether there was any coordination between Trump's campaign team and Russia during the 2016 US election.

Leshchenko showed CNN a six-page document that appears to bear Manafort's signature on a contract and invoice for 501 units of assorted computer equipment, yielding a payment of $750,000.

CNN has been unable to verify the document's authenticity, and Manafort said through a spokesman that he did not recognize the document and that the signature was not his.

But, if genuine, it could be another piece of the puzzle in an investigation launched by Ukrainian authorities looking at whether Manafort and others received millions of dollars in illegal payments from Yanukovych's party.

The document is dated October 14, 2009, during the years when Manafort worked for Yanukovych as a political consultant in Kiev.

The date of the invoice and the $750,000 listed on it match the details logged in an off-the-books ledger, first reported on by the New York Times, which purports to be from Yanukovych's party, listing the series of secret payments now under investigation.

An entry in a ledger of off-the-book payments allegedly from Yanukovych's party shows a payment of $750,000 made to Manafort on October 14, 2009.

Manafort's name appears in the handwritten ledger 22 times as the recipient of payments amounting to $12.7 million, according to anti-corruption authorities in Ukraine.

With this new document, Leshchenko believes he has evidence that Manafort personally invoiced at least one of these payments from Yanukovych's political party.

Anti-corruption investigators in Ukraine have alleged Yanukovych and members of his party ran a corrupt regime. He fled to Russia following a public uprising in 2014.

"So, it looks like Manafort wasn't a political consultant. He was a trader of computer processors," laughs Leschenko.

"But I'm sure it's a fake contract and a fake invoice just to establish -- artificially establish -- a legal basis for a transaction."

A lawyer representing Yanukovych told CNN he had no information on the transfers.

Manafort: 'unfounded, silly and nonsensical'

Jason Maloni, a spokesman for Manafort, denied all the allegations and referenced Ukraine's National Anti-Corruption Bureau (NABU) in his statement.

"The allegations against Paul Manafort are baseless, as reflected by the numerous statements from NABU officials who have questioned the validity of the so-called ledger evidence against Mr. Manafort. Any new allegations by an Serhiy [sic] Leshchenko should be seen in that light and summarily dismissed."

The head of NABU, Artyom Sytnik, told Ukrainian press on November 11, 2016 that "Mr. Manafort does not have a role in this case."

When the ledger surfaced in the midst of the Trump campaign last year, Manafort told CNN allegations of corruption were "unfounded, silly and nonsensical," adding he had never received any cash payments off the book.

He has consistently maintained that this work in Ukraine was perfectly legal. But anti-corruption investigators in Ukraine continue to ask questions about how, when and what amount Manafort and his associates were paid for their work in Kiev.

The FBI is also investigating Manafort over his consulting work in Ukraine and any possible links to Russia. CNN has tried to get a statement from the FBI on investigation of this new document but has received no response.

When CNN asked Leshchenko if he had been contacted by the FBI, he declined to comment. It is unclear if the FBI has seen the document.

On Monday, FBI Director James Comey testified to the House of Representatives on the ongoing investigations of Russian links to the Trump administration and attempts to influence the 2016 election, including the role of Manafort as Trump's campaign manager.

In response, Manafort issued a statement denying "any involvement with Russian government officials to undermine the interests of the United States."

Manafort has previously suggested that the Ukrainian allegations of corruption were part of an attempt by Leshchenko to blackmail him with compromising information about him and Trump. Leshchenko denied those allegations to CNN.

Those allegations arose from text messages allegedly obtained from a hack of an iPhone belonging to Manafort's daughter.

Leshchenko denies he had anything to do with the messages, telling CNN it was a "big surprise" that he was caught up in the scandal and insisting he never wrote any messages to "Manafort, his daughter or any members of his family."

The details

The contract and invoice purport to be between the Ukrainian office of Davis-Manafort -- a company that Manafort partly owns -- and Neocom Systems Ltd.

Details about Neocom Systems are scarce. The document showed Neocoms's address as 1 Mapp St Belize City, which houses several offshore companies, including some named in the Panama Papers.

The address belongs to a company named International Corporate Services.

When contacted, the company's managing director, Lizette Ortiz, said Neocom had been dissolved in 2014 and was no longer a legal entity in Belize.

"We also confirm that we never managed the affairs of this company nor did they physically conduct their business from or within our offices.

"We confirm that we were merely the company's Registered Agent/Registered Office (which is a requirement by law for each company to have) at the time of incorporation and never gave them permission or received compensation from them to advertise that they physically had an office or physical presence at our address nor the usage of our telephone numbers or email addresses and if so they would have been doing so illegally," Ortiz said in an email to CNN.

A report by ICP Credit describes Neocom as an offshore company, and gives almost no other details, other than it was established in 2005 with $50,000 of capital.

CNN was unable to reach the company through the phone number the report listed.

But the contract and invoice suggest an around-the-world paper trail -- Neocom lists its bank accounts in Kyrgyzstan and Germany.

The "Asiatic Universal Bank" listed as Neocom's bank was nationalized by the Kyrgysztan government in 2010 amid allegations of money laundering.

In the document, Davis-Manafort lists its Wachovia bank account in the US to receive money for the invoice.

A locked safe

The document was discovered after the new tenants of an office in Kiev cracked open a locked safe that had been left behind by the previous tenants -- a group of American lobbyists, according to Leshchenko.

The document might have been forgotten as an odd but otherwise innocuous office relic, if not for what appears to be Manafort's signature, scrawled in black over a blue Ukrainian stamp.

Leshchenko has built a career exposing political corruption in Ukraine, first as an investigative journalist and now as a lawmaker. He often receives tips and documents, sometimes mailed anonymously to his office.

On Tuesday, Leshchenko laid out his case to reporters in a press conference holding up copies of the document and the Party of Regions ledger side by side: "This transaction was a payment for political consultancy according to the Black Ledger. But this fake invoice and fake contract signed by Paul Manafort is not about political consulting. It's about selling of computers."

Asked if he believes this is evidence of money laundering in a CNN interview, he said: "I believe this issue needs to be investigated and this issue has to be checked. But in my experience, this looks like money laundering and wire fraud."

But Phil Griffin, a former lobbyist with Davis-Manafort in Kiev, dismissed claims that the company was trying to cover up a money trail and he cast doubt on the authenticity of the document, saying Leshchenko has "zero credibility".

"I wasn't involved in the accounting. Only the day to day political operations," he told CNN. "Look, it's a photocopy of a 10-year-old document. It's probably completely fabricated. I think it's all made up."

After the interview, Leshchenko offered to drive CNN to the former office of Davis-Manafort in Kiev.There is no sign on the ground floor office, no name on the doorbell and no indication of the political operation that brought Yanukovych into power. Leshchenko called the place an "artifact of corruption" and compared it to Yanukovych's former palace, now transformed into a public park and "Museum of Corruption."

"I think it's very important for people to see what corruption looks like. Not only a virtual reality, but in a reality they can see and feel."