Former Prime Minister Yulia Tymoshenko, an opposition leader, is being tried in Kiev on corruption charges stemming from 2009 deal she helped broker with Gazprom.
That deal ended a contract row that affected all of Europe, though the government of Ukrainian President Viktor Yanukovych, a Tymoshenko rival, claims the deal damaged Ukraine's economy.
Yushchenko, during his latest testimony in Kiev, called on Gazprom officials to appear in court, saying they would shed light on the deal. He said Tymoshenko, his ally during the country's 2004 Orange Revolution, moved too quickly in securing the deal.
A source in the Kremlin told Russia's state-run news agency RIA Novosti that the former president lied and actually wanted to sign the deal himself with Russian President Dmitry Medvedev.
"Moreover, in his later telephone conversations with Dmitry Medvedev, Yushchenko said he fully trusted Yulia Tymoshenko, that she had all the necessary powers and that he would support all agreements reached with Russia," the source was quoted as saying.
Tymoshenko claims that charges against her are politically motivated, a claim supported by Washington.