The Kremlin on Monday accused Ukraine of constantly changing its stance when it comes to issues that have already been agreed at peace talks.
"Contacts continue at an expert level within the framework of the negotiation process", Kremlin spokesperson Dmitry Peskov told reporters on a conference call.
"Unfortunately the Ukrainian side is not consistent in terms of the points that have been agreed", he said.
"It is often changing its position and the trend of the negotiating process leaves much to be desired."
He also added that there was still time for so-called "unfriendly" countries to switch to payments for gas in roubles, declining to disclose information on how many countries have agreed to do so.
At the end of March, Russian President Vladimir Putin signed a decree demanding foreign buyers pay for gas in the Russian currency or else have their supplies cut, a move European capitals rejected and which Germany said amounted to "blackmail".
Peskov said payments for deliveries that took place after Putin's decree took effect were expected in May.
Russia sent its troops into Ukraine on Feb. 24 in what it called "a special military operation." Ukrainian forces have mounted fierce resistance and the West has imposed sweeping sanctions on Russia.
