"Russia initially did not agree to a two-for-three swap, but later changed its mind because they were interested in having their high-ranking intelligence officers back," Jonas Vytautas Zukas told the Ziniu Radijas radio station.

He noted that Nikolai Filipchenko, one of the convicted Russian spies, was a career officer of the Russian Federal Security Service (FSB) and holds the military rank of lieutenant colonel.

"I believe Russia was also very interested in the success of these talks and in that exchange," Zukas said.

In the so-called "spy swap" completed last week, Lithuania released Filipchenko and Sergey Moiseyenko, the other Russian citizen convicted of espionage, in exchange for Lithuanian citizens Yevgeny Mataitis and Aristidas Tamosaitis, and Norwegian citizen Frode Berg.

Zukas noted that the two Lithuanian citizens had been arrested and convicted of spying in Russia later than the Russian citizens in Lithuania.

"If it hadn't been for their officers' arrests on our part, they would have probably watched them (the Lithuanians) for a longer time. But they needed people they could exchange (for the Russian citizens). That was why they handed them (the Lithuanians) such lengthy prison terms, of 14 and 12 years," the advisor told the radio station.

"These are very long (sentences); I don't think they we adequate," he added. "I think it was Russia's response to the arrests of such high-ranking officials in Lithuania."

Zukas said more details about the exchange will be made public "sooner or later".