Russia disregarded international law and Bulgaria will not support an end to sanctions imposed by the EU, Foreign Minister Daniel Mitov has said.

Mitov has told Reuters in an interview that Russia's actions with regard to the Ukraine crisis "can't be left without consequences and the only peaceful instrument is sanctions."

He has opined the economic and other restrictions have to stay "until the Minsk agreements are fully implemented," in a reference to a set of deals aimed at easing down tensions in the country's East.

Mitov's words come against the backdrop of a diplomatic offensive of Russia in the EU's southern and eastern member states with Moscow working to secure support for a reversal of sanctions.

Separately, Bulgarian Prime Minister Boyko Borisov has sought to mend fences with the Russian leadership after three large-scale energy projects with Russian participation were suspended during his two terms of office.

Sofia also seeks to talk Moscow into joining the so-called Balkan Gas Hub project, with Sofia aiming to become a regional distribution center.

"It would be good for us if the economic relationship improved, but there are bigger issues on the table," Reuters quotes Mitov as saying.

Commenting on the escalation of words between the EU and Turkey on the migrant deal reached in March, he has urged Ankara to abide by the agreement to readmit migrants before being granted visa-free travel.

Turkey has previously warned it will stop holding on to the deal and will not curb the migrant inflow into Europe if Brussels does not waive visas for its citizens.

The EU, on the other hand, has been demanding a change in Turkey's anti-terror legislation, a step Ankara has categorically ruled out.