Only 20 out of 200 000 ethnic Bulgarians living in Ukraine have been called up as the partial mobilization is ongoing, Ukrainian Foreign Minister Pavlo Klimkin has said.

Klimkin, who is on a visit to Bulgaria Monday, told his Bulgarian counterpart Mitov that Russian authorities were spreading false information and speculating on the issue of mobilization, with the latter not being carried out on an ethnic basis.

Reports in Russian-language media over the past months suggested Bulgarians in Ukraine were being forced into fighting on Kiev's side in the conflict with pro-Russian rebels in the Donbass region.

Ukraine's top diplomat blamed rebels for violating a ceasefire that has been in force since midnight on Sunday. The Organization for Security and Cooperation in Europe (OSCE) suggests both sides are still taking part in fighting.

At a press conference with Mitov, Klimkin made clear Kiev would help Sofia in its plans to provide ethnic Bulgarians with humanitarian aid.

Against official numbers, some reports suggest the real number of ethnic Bulgarians living in Ukraine could amount to 300 000 - 350 000.

Mitov, for his part, asserted Bulgaria would do "everything" to protect the lives of Ukraine's citizens of Bulgarian origin, though he stopped short of details.

Klimkin is also due to meet Prime Minister Boyko Borisov later on Monday.

According to the Foreign Ministry's press office, the Bulgarian top diplomat also told Klimkin that Ukraine had an important place in Bulgaria's geopolitical priorities, both due to the presence of a significant Bulgarian diaspora and to the linguistic, historic, cultural and religious proximity.