Calls coming from Central Europe for a "reserve border system" at Bulgaria and Macedonia's borders with Greece has been the main issue in a phone conversation of Bulgaria's Prime Minister with his German counterpart Angela Merkel.

Bulgaria's PM has been invited to a forthcoming Visegrad Four meeting in Prague, the Czech Republic, to discuss the proposal of the four countries to boost the role of Sofia and Skopje in the protection of the EU's borders in light of the migrant influx.

The fourt states have suggested the step is needed as Greece is, in their view, largely failing to provide border security, letting tens of thousands of migrants a month cross into Europe.

The latter view is also shared by the European Commission, which earlier this week gave Greece a three-month deadline to improve border security.

Ealier on Sunday, it was reported that Berlin disagrees with the Visegrad Group on the need to establish the "reserve system".

However, the Bulgarian government's press service quotes Borisov as confirming in his conversation with Merkel Sofia's "solidarity" with Athens and its rejection of any steps that do not involve all EU member states and are not a result of collective decision-making.

Borisov has reportedly also urged Merkel to support Bulgaria amid the "complications" caused by a Greek farmers' protest at the Bulgaria-Greece border.