MOSCOW, Dec 20 (Reuters) - President Vladimir Putin on Thursday accused the United States of raising the risk of nuclear war by threatening to spurn a key arms control treaty and refusing to hold talks about another pact that expires soon.

In a news conference that lasted more than three hours, Putin also backed U.S. President Donald Trump's decision to pull troops out of Syria, said British Prime Minister Theresa May had no choice but to implement Brexit and that Western democracy was under serious strain.

The annual event, the 14th of its kind, is used by Putin to burnish his leadership credentials and send messages to foreign allies and foes.

This year, he made clear his biggest worry was what he called a dangerous new arms race, something he accused the United States of stoking by turning its back on arms control.

Washington has threatened to pull out of the 1987 Intermediate-range Nuclear Forces Treaty (INF) which bans Moscow and Washington from stationing short- and intermediate-range, land-based missiles in Europe.

Putin said the move, if it happened, would have unpredictable consequences.

"We are essentially witnessing the breakdown of the international arms control order and (the start of) an arms race," Putin told more than 1,000 reporters.

"It's very hard to imagine how the situation will develop (if the United States quits the INF treaty). If these missiles appear in Europe what should we do? Of course, we'll have to ensure our own security."

Putin has previously said that Russia would be forced to train its own missiles on any European countries that host U.S. rockets.

Related: President Donald Trump meets with Vladimir Putin in Finland

15 PHOTOS President Donald Trump meets with Vladimir Putin in Finland See Gallery President Donald Trump meets with Vladimir Putin in Finland Russia's President Vladimir Putin (R) and U.S. President Donald Trump attend a meeting in Helsinki, Finland July 16, 2018. Sputnik/Alexei Nikolsky/Kremlin via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. HELSINKI, FINLAND - JULY 16, 2018: The national flags of Russia and the United States seen ahead of a meeting of Russia's President Vladimir Putin and US President Donald Trump. Valery Sharifulin/TASS (Photo by Valery Sharifulin\TASS via Getty Images) US President Donald Trump (L) and Russian President Vladimir Putin arrive for a meeting in Helsinki, on July 16, 2018. (Photo by Brendan Smialowski / AFP) (Photo credit should read BRENDAN SMIALOWSKI/AFP/Getty Images) Russia's President Vladimir Putin (R), U.S. President Donald Trump (C) and First lady Melania Trump attend a meeting in Helsinki, Finland July 16, 2018. Sputnik/Alexei Nikolsky/Kremlin via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. HELSINKI, FINLAND - JULY 16: (----EDITORIAL USE ONLY MANDATORY CREDIT - 'KREMLIN PRESS OFFICE / HANDOUT' - NO MARKETING NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS----) (L-R) US First Lady Melania Trump, US President Donald Trump, Russian President Vladimir Putin, Finnish President Sauli Niinisto and his wife Jenni Haukio pose for a photo during their meeting in Helsinki, Finland on July 16, 2018. (Photo by Kremlin Press Office / Handout/Anadolu Agency/Getty Images) Russia's President Vladimir Putin (R), U.S. President Donald Trump (C) and First lady Melania Trump attend a meeting in Helsinki, Finland July 16, 2018. Sputnik/Alexei Nikolsky/Kremlin via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. U.S. President Donald Trump meets with Russia's President Vladimir Putin in Helsinki, Finland, July 16, 2018. REUTERS/Kevin Lamarque U.S. President Donald Trump and Russia's President Vladimir Putin shake hands as they meet in Helsinki, Finland July 16, 2018. REUTERS/Kevin Lamarque Russia's President Vladimir Putin (R) and U.S. President Donald Trump attend a meeting in Helsinki, Finland July 16, 2018. Sputnik/Alexei Nikolsky/Kremlin via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. HELSINKI, FINLAND JULY 16, 2018: US President Donald Trump (L) and Russia's President Vladimir Putin give a joint news conference following their meeting at the Presidential Palace. Valery Sharifulin/TASS (Photo by Valery Sharifulin\TASS via Getty Images) U.S. President Donald Trump and Russian President Vladimir Putin react at the end of the joint news conference after their meeting in Helsinki, Finland, July 16, 2018. REUTERS/Leonhard Foeger U.S. President Donald Trump receives a football from Russia's President Vladimir Putin during their joint news conference after a meeting in Helsinki, Finland, July 16, 2018. REUTERS/Kevin Lamarque Russia's President Vladimir Putin (C), U.S. President Donald Trump (R) and First lady Melania Trump pose for a picture with a football during a meeting in Helsinki, Finland July 16, 2018. Sputnik/Alexei Nikolsky/Kremlin via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. HELSINKI, FINLAND JULY 16, 2018: US First Lady Melania Trump, Russia's President Vladimir Putin, and US President Donald Trump (L-R) after a news conference at the Presidential Palace. Valery Sharifulin/TASS (Photo by Valery Sharifulin\TASS via Getty Images) U.S. First Lady Melania Trump holds a football thrown to her by U.S. President Donald Trump during his joint news conference with Russia's President Vladimir Putin after a meeting in Helsinki, Finland, July 16, 2018. REUTERS/Kevin Lamarque Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

Another U.S.-Russia treaty, the New START pact, which limits the number of deployed strategic nuclear warheads each side can have, expires in 2021. Putin said he was worried that Washington didn't appear to be interested in discussing its future.

"No talks on extending this are yet being held. Are the Americans not interested, do they not need them? Ok, we'll survive and will ensure our own security, which we know how to do. But in general, this is very bad for humankind because it takes us closer to a dangerous threshold."

GROWING THREAT

The Russian leader, who said Moscow had developed nuclear weapons which he believed gave it an edge over other countries, warned the threat of a nuclear conflict was growing as a result of the U.S. moves. He also cited the dangerous tendency of lowering the threshold for using nuclear weapons and the idea of using ballistic missiles with conventional warheads.

"If, God forbid, something like that were to happen, it would lead to the end of all civilisation and maybe also the planet," said Putin.

But though Putin criticized Washington, he made clear that he still hoped to meet Trump sometime soon, saying it was important the two leaders discussed issues such as arms control.

Related: Russia prepares to re-elect Vladimir Putin

15 PHOTOS Russia prepares to re-elect Vladimir Putin See Gallery Russia prepares to re-elect Vladimir Putin Veronika Baikova, 41, Russian citizen who plans to boycott the upcoming presidential election, poses for a picture as she visits her relatives in the village of Yukhovichi, Belarus, February 7, 2018. "Me and my friends will not go to the polling station - we don't believe in this election," said Baikova. "But if I go to the polls, I will vote against everyone and especially against Putin. His policies have not given us anything good over the years". REUTERS/Vasily Fedosenko Vasily Slonov, 48, artist and supporter of presidential candidate Pavel Grudinin, poses for a picture inside his workshop in Krasnoyarsk, Russia, February 8, 2018. "I don't think that this will be Putin's final term in office. In fact I see a sort of messianic energy in Putin," said Slonov. "He?s not just any other person, but something of an instrument in God's hands. He's not simply a politician." REUTERS/Ilya Naymushin Natalia Dementieva, 44, an accountant who is currently unemployed and supporter of presidential candidate Ksenia Sobchak, poses for a picture in Moscow, Russia, January 14, 2018. "She speaks the truth, openly. She doesn't lie. She raises issues which are taboo under our government. For example, Crimea. Which means she's not afraid," said Dementieva. "The idea that things have improved, I'm not seeing it. Things have gotten worse. They have started to control what people think even more than during the Communist era. People are afraid again... Ksenia is a politician of the future." REUTERS/Maxim Shemetov Yulia Dyuzheva, 22, student and supervisor of the "SUPERPUTIN" exhibition and supporter of presidential candidate Vladimir Putin, poses for a picture in Moscow, Russia, January 14, 2018. "I'd draw your attention to the colossal support of young people for the current government. As a representative of the younger generation, I can say that for us, young Russians, all the doors are open. Everyone is able to grab the opportunities presented and make the most of themselves, in whatever town or region," said Dyuzheva. "For me, Russia's current leader is taking the country down a very rational path, a path based on justice, openness, one which places a stress on values and traditions, and takes a very clear position on the global stage." REUTERS/Maxim Shemetov Andrei Vorontsov, 42, ataman of a local Cossack society and supporter of presidential candidate Vladimir Putin, poses for a picture in Mikhaylovsk town in Stavropol Region, Russia, February 21, 2018. "It's hard to say what the future will look like," said Vorontsov. "The way I see it, people can make assumptions, but it's God that decides. The way He rules it, that's how things will be." REUTERS/Eduard Korniyenko Alexander Reshetnyak, 42, Cossack ataman and supporter of presidential candidate Vladimir Putin, poses for a picture in Stavropol, Russia, February 16, 2018. "Of course, among the candidates right now, in my opinion no one can compete with our current president," said Reshetnyak. "There's no alternative. So yes, maybe these are elections without a genuine choice." REUTERS/Eduard Korniyenko Tanzurun Darisyu, 51, supporter of presidential candidate Vladimir Putin and head of a private farm located in Kara-Charyaa area in the south of Kyzyl town, the administrative centre of the Republic of Tyva (Tuva region), poses for a picture inside her yurt in Southern Siberia, Russia, February 14, 2018. "We, the Arat people, farmers, need to be able to be confident about what tomorrow will look like. We need this in order to expand and develop our farmsteads." said Darisyu. REUTERS/Ilya Naymushin Anastasia Shevchenko, 38, head of the election campaign team for presidential candidate Ksenia Sobchak, poses for a picture in Rostov-on-Don, Russia, February 13, 2018. "I want change, of course. First of all, in how this country is run. I hope that something will change. That's why I completely support my candidate's programme," said Shevchenko. "But at the same time, I completely understand that these elections don't decide anything. I just want new people in our politics." REUTERS/Sergey Pivovarov Andrei Lukinykh, 46, engineer and supporter of presidential candidate Vladimir Putin, poses for a picture in Yevpatoriya, Crimea, February 20, 2018. "I am going to vote, because I want stability. As the saying goes, you don't change your horses when crossing a river (at a turning point, during a difficult time)," said Lukinykh. "Unlike the others, my candidate can provide the stability that's needed." REUTERS/Pavel Rebrov Vitaly Bespalov, 26, journalist and supporter of presidential candidate Ksenia Sobchak, poses for a picture, showing a tattoo depicting Sobchak on his arm, in St. Petersburg, Russia, February 14, 2018. "I am 26. Eighteen of those years, all of my conscious life, I have lived under a single president," said Bespalov. "I have been waiting too long for change." REUTERS/Anton Vaganov Alexei Gruk, 45, mechanic and supporter of presidential candidate Pavel Grudinin, poses for a picture in St. Petersburg, Russia, January 31, 2018. "The most important thing for me is that our foreign policy stays the same," said Gruk. "To hell with the sanctions? So what if they don?t bring foreign stuff here anymore? As if that means we have to give up. I don't care." REUTERS/Anton Vaganov Marina Kalinichenko, 54, a maths teacher and supporter of presidential candidate Vladimir Putin, poses for a picture in Yevpatoriya, Crimea, February 21, 2018. "As Mark Twain said, 'if voting made a difference, they wouldn't let us do it'. I really do support my candidate, but I also realise that it's the oligarchs that truly decide, who becomes president." said Kalinichenko. REUTERS/Pavel Rebrov Maxim Gubsky, 21, student and supporter of presidential candidate Vladimir Zhirinovsky, poses for a picture in Stavropol, Russia, February 20, 2018. "I don't expect Putin to change things, and history proves this to be true," said Gubsky. "Little has changed over the past twenty years... to see progress in my life, we need a new president, new ambitions and new decisions." REUTERS/Eduard Korniyenko Svyatoslav Lomakin, 19, student and supporter of presidential candidate Vladimir Zhirinovsky, poses for a picture in Tula, Russia, January 26, 2018. "There's something of a shift, but I want real changes, which will improve our lives," said Lomakin. "I want there to be more opportunities for the young, so they can really make the most of their lives." REUTERS/Maxim Shemetov Vitaly Zubenko, 45, lawyer and supporter of presidential candidate Grigory Yavlinsky, poses for a picture in Stavropol, Russia, February 19, 2018. "When the people in power don't change, it's the foundation for corruption, it's what corruption needs to remain undefeated," said Zubenko. "We see that for the past four years, real incomes have been falling. A few people might have seen things improve, but overall, the population, the country, business, entrepreneurs, all economic structures, they're just about surviving. Things are getting worse and worse, unfortunately... We have to change the constitution, so that power never again finds itself in the hands of a single person." REUTERS/Eduard Korniyenko Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

With relations between Moscow and Washington strained by everything from Ukraine to hacking allegations, Putin said he did not know when a meeting with Trump might take place however.

Putin won a landslide re-election victory in March, giving him six more years in power. Although he faces no serious political threat for now, plans to sharply raise the pension age saw his approval rating fall to below 60 percent for the first time in five years.

(Additional reporting by Gabrielle Tetrault-Farber, Polina Ivanova, Tom Balmforth, Darya Korsunksaya, Masha Tsvetkova, Elena Fabrichnaya, Maria Kiselyova, Ekaterina Golubkova, Vladimir Soldatkin, Andrey Ostroukh; Writing by Andrew Osborn; Editing by Jon Boyle)

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