"The issue has been brought up, studied by experts and a report has been made to the government," Vitaly Radetsky told reporters after talks Tuesday with Sagadat Nurmagambetov, the Kazakh defense minister.
But he added, "After the government and president approve a decision, we can start talking about agreements. For the moment this is a far-fetched fantasy."
The possible sale of Ukrainian bombers was first raised this week by a Russian Defense Ministry source, quoted by Interfax.
Ukraine has accumulated debts to Russia of about $1.2 billion for gas supplies.
It agreed at talks in Kiev last week to pay about one-third of the sum within the next three months.
Radetsky also rejected any suggestion that Ukraine could join a collective security pact signed in 1992 and made up of seven former Soviet republics.
"We have to look to our military doctrine, which says we can be a part of no bloc. And if that is the case, there can be no talk about collective security," he said. "We can only come to agreements on a bilateral level."
The chief of staff of Ukraine's armed forces suggested in an interview this week that Ukraine would find it cheaper to finance its defense with an agreement on collective security with other former Soviet republics.
President Leonid Kuchma, who was elected last month, has called for closer economic links with Russia and other former Soviet republics. But he has ruled out defense cooperation, considered too sensitive in a country ruled by Moscow for three centuries.
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