Von der Leyen
Leaders of the four are due to discuss the issue at a meeting on June 11. But experts and officials are sceptical they will bridge their differences.
"We don't expect to see a strong unified front as we did during the migrant crisis," said Vit Havelka, an analyst at the Prague-based Europeum think tank.
Greek Prime Minister Kyriakos Mitsotakis has sent letters to top EU officials protesting over Turkey's activities in the Eastern Mediterranean and the publication in the Turkish government's gazette of a map outlining areas of the Greek continental shelf where Turkish Petroleum Corporation (TPAO) has applied for exploration permits.
Greece is planning purchases worth millions of euros to equip the Hellenic Police's (ELAS) riot force with shields, helmets, uniforms, bullet-proof vests and other supplies, including large quantities of tear gas, in anticipation of a fresh push on the northeastern border with Turkey by undocumented migrants and refugees trying to force their way into the European Union.
Greece on Wednesday welcomed a plan proposed by the European Commission to disburse 750 billion euros in grants and loans to European Union countries to help them recover from the economic impact of the coronavirus.
Under the proposal, Greece is expected to receive 32 billion euros from the new recovery fund over three years - 22.5 billion in grants and 9.4 billion in loans.
She stated that Serbia was "part of the Europe team" which collected over 4 billion dollars for the fight against COVID-19.
"Western Balkans belongs to the EU and no one in the EU questions this. I firmly believe that the EU has a special responsibility to help its partners in the region," von der Leyen said.