By Polina Nikolskaya and Andrew Osborn
MOSCOW (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.
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."
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 criticised 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.
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)