(CNN) Ukrainian prosecutors want to question Paul Manafort in connection with a corruption investigation and have made repeated requests for assistance from US authorities, CNN has learned.

Prosecutors in Kiev said they have made seven separate appeals over the past two years for help in questioning President Donald Trump's former campaign manager, including letters to FBI Director James Comey and US Justice Department officials. Ukrainian officials said the US has not responded to those requests.

Under a "mutual legal assistance" treaty, the two countries have agreed to regularly assist each other in law enforcement efforts, such as gathering statements and other evidence for prosecutions.

US authorities confirmed to CNN that the requests were received but declined further comment.

Manafort served as Trump's campaign chief until being pushed out in August 2016.

The official requests from a special prosecution unit in Kiev started in December 2014, and involve a corruption case targeting Ukraine's former Justice Minister Oleksandr Lavrynovych.

Manafort has not been charged with a crime. Prosecutors want him to testify, Ukraine's prosecutor for special investigations Serhiy Gorbatyuk said.

Prosecutors allege that Lavrynovych illegally diverted more than $1 million in government funds to a prominent New York City law firm — Skadden, Arps, Slate, Meagher & Flom.

Lavrynovych had hired Skadden to review the 2011 jailing of former Ukrainian Prime Minister Yulia Tymoshenko, who had received a seven-year sentence for allegedly harming Ukraine's interests in gas supply negotiations with Russia.

Tymoshenko was the main political rival of the Kremlin-backed President Viktor Yanukovych, whom Manafort advised until Yanukovych was deposed in 2014. Tymoshenko was released from jail at the same time that Yanukovych was ousted.

Many saw her sentencing as politically motivated by the pro-Russian government.

In 2012, as a result of the sentencing, Ukraine faced the possibility of an unfavorable ruling from the European Court of Human Rights, and US lawmakers were considering sanctions.

In response to the deteriorating international climate, Ukrainian prosecutors say, Manafort drafted a public relations strategy that included hiring Skadden to review the Tymoshenko case and show the conviction had a sound legal basis.

But a spokeswoman for the US State Department in 2012 said the report Skadden eventually came out with "was incomplete and doesn't give an accurate picture."

Prosecutors told CNN that Ukraine's Justice Ministry had signed an agreement with Skadden to provide legal services for the equivalent of only $12,000, the legal limit above which it would have been necessary to hold a public competition to award a government contracting job.

Prosecutors provided CNN with the details of a bank transfer that took place on May 30, 2013 to a Citibank account allegedly belonging to Skadden from a Justice Ministry bank account.

The document shows the ministry wired Skadden more than $1 million -- $1,075,381.41. It is this payment that prosecutors charge was a misappropriation of government funds.

"We believe they wanted to avoid the time consuming competition they would have had to organize to hire the law firm legally, so they drew up the undervalued contract and probably arranged to pay the real fee in cash," Gorbatyuk told CNN, speaking in Russian.

However, when Tymoshenko's legal team publicly pointed out that the government could not have secured Skadden's services for just $12,000, Skadden asked Ukrainian officials to draw up a second contract that reflected a more realistic fee, prosecutors allege.

Asked to comment on its involvement in the matter, Skadden released a statement to CNN: "We have been and will continue cooperating with appropriate requests."

Members of Gorbatyuk's office showed CNN a record of seven separate occasions when they asked the US authorities for help.

The first letter was sent in December 2014 to the US Justice Department's Office of International Affairs, and was a request to question Skadden partner Gregory Craig, who also served as a White House counsel in the Obama administration.

In December 2015, Gorbatyuk's office sent another letter to the Department of Justice asking to question Manafort.

The trigger for the request was two emails prosecutors found. The contents of those emails were provided to CNN: One between Craig and Manafort where Craig asked Manafort to help secure paperwork from Ukraine needed for Skadden's report, and another between a Skadden employee and a Ukrainian official that mentioned Manafort's presence at a meeting with the Justice Minister.

Prosecutors also showed CNN documentation they sent to the DOJ in which they told the US authorities that their investigation had "established that the well--known American political strategist Paul Manafort is implicated in the relationship between the Skadden Arps. firm and the Justice Ministry of Ukraine." Of Manafort, the letter said he "was likely the person who advised representatives of the former Government of Ukraine to hire the law firm and was present during talks about this issue."

Manafort declined to provide comment for this story.

The final letter was dated September 2016: Ukraine's Prosecutor General, the equivalent of a US Attorney General, sent a letter directly to FBI director James Comey asking for clarification for why the US authorities would not help.

Some of the attempts to question Manafort coincided with a time period when the FBI grappled with the issue of whether to subpoena Manafort in its separate investigation surrounding his business dealings in Ukraine, last summer. Comey has faced criticism for taking a different approach on legal matters relating to the Hillary Clinton campaign after he controversially sent a letter to Congress announcing the FBI's renewed interest in her use of a personal server during her time as Secretary of State just 11 days before the election.

Manafort currently faces an FBI investigation over millions of dollars' worth of payments he allegedly received while working for Yanukovych. Manafort has denied those claims.