LONDON, Dec 1 (Reuters) - NATO Secretary-General Jens Stoltenberg warned Russia not to use force against neighbour Ukraine on Wednesday and told Reuters that Moscow knows it would pay a high price through sanctions and other steps by the West for any aggression. Ukraine, a former Soviet republic that now aspires to join the European Union and NATO, has become the main flashpoint between Russia and the West as relations have soured to their worst level in the three decades since the Cold War ended. "We all made it very clear that there will be a high price to pay and, and sanctions is one of the options," Stoltenberg said in an interview at the Reuters Next conference. Russia says it has no intention of invading Ukraine. "I think it"s quite obvious that Russia already knows that they will pay a higher price," Stoltenberg added. The NATO chief said fresh economic sanctions on Russia - which the EU and the United States, along with Britain and Canada could impose - were one powerful tool. President Vladimir Putin has said that Russia would be forced to act if U.S.-led NATO placed missiles in Ukraine that could strike Moscow within minutes. Stoltenberg said it was public knowledge that allies provide military support to Ukraine and that this was defensive and in line with all allies" international obligations. Ukraine is considered a close partner of NATO, but is not yet a member of the 30-nation Western alliance. NATO Secretary General Jens Stoltenberg speaks during the NATO Foreign Ministers summit in Riga, Latvia December 1, 2021. REUTERS/Ints Kalnins NATO Secretary General Jens Stoltenberg speaks during the NATO Foreign Ministers summit in Riga, Latvia December 1, 2021. REUTERS/Ints Kalnins "NOT TRANSPARENT" Stoltenberg rejected accusations the alliance was provoking Moscow with military exercises in the Black Sea, and blamed Russia in turn for having used military drills as a disguise for attacking neighbouring countries. In contrast to Russia, NATO always followed regulations on inviting foreign observers to its drills, said Stoltenberg, a former Norwegian prime minister who is set to leave the top NATO post at the end of next year. He declined to comment on the search for his successor. "The problem with Russia is that they are not transparent, that they have a very aggressive rhetoric and a track record showing they have used military exercises before as a disguise for aggressive actions against neighbours," he said. Stoltenberg said that the Western alliance was working closely to protect allies against new Chinese and Russian missiles that can reach Europe and North America. "Russia, but also China, now invest heavily in nuclear-capable systems that can reach all NATO countries," Stoltenberg said. "It is something we have to take very seriously," he added, citing Russian investment in extremely long-range, high-speed, manoeuvrable missiles. To watch the Reuters Next conference please register here https://reutersevents.com/events/next/