NATO members and partners have launched their largest ever joint exercise since the Cold War in Poland. The two-week drill kicked off on Monday in Warsaw.

It comes at a time when central and eastern European nations are seeking strong security guarantees amid concerns about Russia.

Codenamed "Anaconda-2016", the exercise will involve some 31 thousand troops from Poland, the US and 17 other NATO member nations, as well as from five partner nations.

Russia called the move "unjustified", adding that any presence of NATO troops close to its borders is a threat to its security.

"The Anaconda military exercise aims to test the capabilities of the NATO member states in defending the territory of the eastern flank," said Antoni Macierewicz, Polish defence minister.

"The United States Army and the United States military and really, the United States of America, have a single purpose in Operation Anaconda and that is to bring all of us together to demonstrate that we are shoulder to shoulder with the Polish people," said Mark Milley,general of US Army Chief of Staff. 

"We are shoulder to shoulder with the Polish Army and that we are shoulder to shoulder with NATO to ensure that all of the countries of NATO remain free and independent."