Bulgarian Prime Minister Boyko Borisov has said he talked to Turkish President Recep Tayyip Erdogan while on a visit to Istanbul.

The two discussed the migrant crisis which Erdogan pointed out that has "grown into a global problem", according to the government's press office. Erdogan called on the EU to help relieve the burden placed on Turkey in tackling the crisis and to consider immediate steps at the next EU leaders' meeting.

Borisov asserted Erdogan Turkey could count on Bulgaria's full support.

Over the past weeks Sofia has been among the most vocal supporters of a proposal by Turkey that a safe zone should be established in Northern Syria and also that Brussels should allocate funding to Ankara so that the migrant influx is managed more effectively.

As of Wednesday Erdogan was not in Istanbul, where Borisov met with his counterpart, Ahmet Davutoglu.

Borisov later wrote on Facebook that in the phone conversation he had extended his condolence to the Turkish head of state after the twin attack in Turkey's capital Ankara which took 97 lives and wounded hundreds on Saturday morning.

"With Mr Erdogan we are unanimous that effective border control, both on behalf of the EU and on transit countries, is of key importance to security and to the prevention of such tragic incidents," the Bulgarian Prime Minister noted, stressing the importance of bilateral cooperation on border security.