Russian Prime Minister Vladimir Putin has expressed outrage at reports that Ukraine supplied arms and technicians to Georgia before the recent war.
After talks near Moscow with his Ukrainian counterpart Yulia Tymoshenko, he said that if such reports were confirmed, they constituted a "crime".
Ms Tymoshenko said she was confident the reports would prove unfounded.
The two premiers agreed to soften the blow of switching to market rates for supplying Russian gas to Ukraine.
While they did not mention any figures at their meeting, they signed an energy memorandum meant to act as the basis for future gas negotiations.
"The parties confirmed their willingness to establish a gradual transition to market prices within three years," said Ms Tymoshenko.
"We have come to the conclusion that our countries do not need shock therapy."
Russia briefly cut gas supplies to Ukraine earlier this year, accusing it of failing to pay a debt of about $1.5bn which Kiev insisted it had paid.
'A crime'
Pointing to a report published in Russia's Izvestiya newspaper this week, Mr Putin said Ukrainian weapons and military experts may have been used in combat against Russian troops during the brief war with Georgia in August.
"If this is confirmed, this will be what I have called a crime, because this is termed 'direct involvement in an armed conflict', pitting the Russian and Ukrainian peoples against each other," he added.
If Russia received proof of Ukraine's military involvement, Moscow would "build its relations accordingly with those who allowed this to happen", he said.
Ms Tymoshenko said she was confident that "such facts [would] not be confirmed".
Correspondents say Mr Putin is aware that Mrs Tymoshenko is not responsible for Ukraine's defence policy, and that his criticism amounted to an attack on her political rival, President Viktor Yushchenko.
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