"Experts estimate that in February it will be too late," Kasyanov told journalists.
On Friday Moscow will host talks between Kasyanov and his Belarussian counterpart Sergei Sidorsky.
"We have to sign an agreement and to pass it through legislative procedures in order not to miss the last chance of introducing the Russian ruble as Belarus' only currency from January 1, 2005," the Russian Prime Minister said.
The forthcoming talks will also touch upon cooperation in the energy sphere, he added.
"We will discuss aspects important both for Belarus and for us," he said. Russia is interested in signing an agreement on creation of a joint Russian-Belarussian gas transporting venture, as well as in contracts on gas transportation and deliveries, Kasyanov explained.
The Prime Minister expects that at the talks the parties will achieve progress both in the ruble introduction and in the gas issue.