Nato chief Jens Stoltenberg on Wednesday demanded Moscow prove it is complying with a landmark Cold War nuclear arms reduction treaty, as concerns grow over a new Russian missile system. Nato defence ministers will discuss the issue at a two-day meeting in Brussels, after the alliance's July summit declaration said the Russian 9M729 missile system raised "serious concerns". Stoltenberg said the 1987 accord signed by US president Ronald Reagan and Soviet leader Mikhail Gorbachev remained a "cornerstone of European security".