4 NATO countries are calling for a defense line along Europe’s 700-mile border with Russia and Belarus

• The Baltic states and Poland have called for a defense line along Europe's border with Russia, per Reuters.
• They said it would protect the EU from Russia's "military" and "hybrid" threats.
• Frontline NATO countries are facing intensifying Russian hybrid warfare threats.

NATO member states Latvia, Lithuania, Estonia, and Poland have urged the EU to build defensive infrastructure along its 700-mile-long border with Russia and Belarus, according to Reuters.

The leaders of the four countries, which all share a land border with either Russia or Belarus, called for "extraordinary" measures to protect them and their allies.

"Building a defense 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," they said in a joint letter sent to the EU chairman, per Reuters.

They said the plan should be discussed at a two-day summit in Brussels starting on June 27, during which time EU leaders will discuss defense funding, among other items.

Given the size and cost of the effort — which some EU diplomats estimate would be about $2.7 billion — bloc-wide action is needed to support it both politically and financially, the letter said, per Reuters.

It didn't specify exactly what military or civilian means should be deployed.

Front-line NATO countries have faced intensifying Russian hybrid warfare threats, unconventional methods that Russia seems to be using that blur the line between war and peace and that fall into what is called the "gray zone."

Recent examples include Russia's coast guard removing buoys demarcating the territorial waters between Russia and Estonia on the Narva River in May, a day after a leaked Russian proposal outlined plans to redraw Russia's territorial waters with Estonia, Lithuania, and Finland.

The Baltic region has also experienced increased jamming of commercial aircraft's satellite navigation systems, which seems to be coming from Russia.

Meanwhile, EU officials have accused Russia and Belarus of driving migrants toward Lithuania's border with neighboring Poland.

Influence operations have also targeted all three Baltic countries, with Estonia experiencing a rise in sabotages that damaged an undersea gas pipeline and telecommunications cables between the country and Finland in October.

In response to these threats, and the threat of a potential Russian invasion, NATO and European countries close to Ukraine and Russia have started drafting their own defensive plans to protect their borders.