"NATO is a union based on values and unity. Without that unity, the union risk losing its effectiveness or relevance altogether," the Lithuanian president told journalists in Warsaw after meeting with the leaders of eight other Central and Eastern European countries to discuss the upcoming NATO summit in July.

The summit will take place amid ongoing tension in transatlantic relations, with Washington's decision to impose imports duties on metals from European countries and its decision to pull out of Iran's nuclear deal causing major disputes between the European Union and the United States.

Unity is undermined not only by problems in transatlantic relations but also by territorial disputes between Turkey and Greece, Grybauskaitė underlined.

"We must be open and speak not only about external challenges but also about the internal ones," the president said.