NATO Secretary-General Jens Stoltenberg says that any use of chemical or nuclear weapons will totally change the nature of the war in Ukraine. It will be absolutely unacceptable.<br />''<br />''Stoltenberg spoke during a visit to the long-planned Cold Response drill in his native Norway and called Russia's military operation  in Ukraine a watershed moment. He also regretted that Moscow had declined to observe the drill, saying that NATO always invite other countries to observe.<br />''<br />''The drill taking place in southeastern, central and northern Norway includes around 30,000 troops from 27 countries. Non-NATO members Finland and Sweden are also participating. <br />''<br />''The exercise started March 14 and ends April 1.<br />''The first Cold Response exercise was held in 2006. It is conducted every two years.<br />''<br />