The Kremlin accused Ukraine on Monday of constantly changing its stance on issues that have already been agreed to 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. It is often changing its position and the trend of the negotiating process leaves much to be desired."

Ukrainian President Volodymyr Zelenskyy said earlier in an interview that the elimination of Ukrainian troops in Mariupol would put an end to any negotiations and called on the west to immediately provide heavy weapons.