NATO is launching its biggest ever exercise in Poland as central and eastern European nations are seeking strong security guarantees among concerns about Russia's assertiveness and actions.

The Anaconda-16 exercise launched Monday in Warsaw will involve some 31,000 troops from Poland, the U.S. and 17 other NATO member nations and from five partner nations.

It is held just weeks before NATO holds a crucial summit in Warsaw expected to decide that significant numbers of NATO troops and equipment will be based in Poland and in the Baltic states. Those countries are particularly concerned about the armed conflict in Ukraine, where Moscow supports separatists.

The exercise runs through June 16 and is to test cooperation between allied commands and troops in responding to military, chemical and cyber threats.