The Left party, which is not part of Sweden's coalition but has 21 seats in the Swedish Parliament, is accused of sending money to the Borotba, a radical left wing group operating in Ukraine with links to President Vladimir Putin and separatists fighting against government forces in eastern Ukraine.
According to Sweden's Aftonbladet newspaper, which broke the story on Wednesday, 219 000 kronor ($25,397) was moved from a bank account linked to the Left Party's aid organisation VIF, to a private bank account in the name of an ex-girlfriend of Borotba's leader, Victor Shapinov.
In an interview with the newspaper the woman said: "He told me to just take the money, because I have a foreign currency account at the bank", adding that she "asked no questions" at the time about what it was for.
The paper speculates that the Left Party may have been tricked by Borotba into thinking the money was bound for humanitarian work in the region.
Shapinov has previously worked as a political consultant for Vladimir Putin's United Russia party. He moved to Ukraine in 2006 and founded the Borotba movement in May 2011. He was arrested in November 2014 in the former Soviet Republic of Moldova amid rumours he was trying to promote Russian separatism in the state and even overthrow the political regime there, although Moldovan authorities have never made public the reason he was held.
The Left Party has previously admitted to collaborating with Borotba, but Aftonbladet understands that any cooperation ceased in December 2014.
The Local has contacted The Left Party for comment.
News of the scandal broke as Sweden's Prime Minister Stefan Löfven visited the Ukranian capital, Kiev, and offered the country a $100 million loan (850 million kronor).
A press release from the Swedish government on Wednesday said: "This loan, together with our political support for the country's autonomous path, independence and territorial integrity, will help to strengthen Ukraine.”