In a letter to the chairman of the EU to be discussed at a summit in Brussels starting on Thursday, the leaders of the four countries that share borders with Russia and Belarus said the project, to protect the 27-nation bloc of 450 million people, would also need the financial support of all members.

"Building a defence infrastructure system along the EU external border with Russia and Belarus will address the dire and urgent need to secure the EU from military and hybrid threats," said the letter of the four leaders, seen by Reuters.

Hybrid threats refer to a combination of military and nonmilitary as well as covert and overt means, including disinformation, cyberattacks, economic pressure and the pushing of migrants across borders.

"The scale and costs of this joint endeavour require a dedicated EU action to support it both politically and financially," the letter said.

Some EU diplomats estimated the cost of building such a defensive line on the ground along the 700 km EU border with Russia and Belarus at around 2.5 billion euros ($2.67 billion).

European investment in defence and its financing will be one of the main topics of discussion among EU leaders at the summit, as Russia's war against Ukraine has entered its third year and Moscow is stepping up hybrid operations against the West.

The call for a jointly funded ground defence line on the eastern border of the European Union comes on top of an earlier initiative by Greece and Poland to create an EU air defence system, modelled on the Israeli Iron Dome, that would coordinate the now separate air defence systems of EU countries.

"Extraordinary measures need to be employed as the EU's external border must be protected and defended with military and civilian means," the letter of the four countries said.

The letter said the planning and execution of the defence line on the EU's eastern border should be done in coordination with NATO and its military requirements.