WARSAW, Poland (AP) — Five ex-presidents from Poland and Ukraine have made a joint appeal for reconciliation between their nations as growing nationalism in both societies and provocations from Russia have strained the alliance.

Poland for years was one of the strongest advocates in the European Union and NATO for bringing Ukraine into the Western fold, a stance that infuriated Moscow. The alliance, however, has been strained in recent times, mostly over disagreements rooted in historical grievances.

The ex-presidents said they were concerned about the strains and that unity is needed today given "the challenge of Russian aggression to Ukraine."

The appeal was presented at a news conference in Warsaw by the two former Polish presidents, Aleksander Kwasniewski and Bronislaw Komorowski, and also signed by Leonid Kravchuk, Leonid Kuchma and Viktor Yushchenko of Ukraine.