Haftar said his Libyan National Army (LNA) was accepting a "popular mandate" to rule over the country, apparently brushing aside the civilian authorities which nominally govern eastern Libya.
"In Moscow, we remain convinced that the only possible resolution in Libya can be through political and diplomatic communication between all parties, above all those in conflict," said Kremlin spokesman Dmitry Peskov said.
"Russia remains in contact with all participants in the Libyan process. We believe that there are no other ways to resolve the Libyan problem."
Libya has been split since 2014 between areas controlled by the internationally recognized Government of National Accord (GNA) in Tripoli and the northwest, and territory held by eastern-based forces in Benghazi.
Haftar is supported by Russia, the United Arab Emirates and Egypt. The GNA is backed by Turkey.
The conflict escalated sharply this month, with fierce fighting on several different fronts in the west of the country despite urgent calls from the United Nations and aid agencies for a truce to tackle the coronavirus crisis.
Russia described Monday's power grab by Haftar, whom Moscow supports, as "surprising," state news agency RIA cited a foreign ministry source as saying.
Most important was for the military and political decisions reached at a conference in Berlin in January to be implemented by Libyans themselves, with the assistance of the international community, the source said.
The EU's top diplomat and the foreign ministers of Germany, France and Italy made a joint call on Saturday for a humanitarian truce in Libya, saying all sides must resume peace talks.