By Nandita Bose and Guy Faulconbridge
WARSAW/MOSCOW (Reuters) - U.S. President Joe Biden said on Tuesday Ukraine "stands strong" a year after the Russian invasion and that Moscow would never defeat its neighbour, after the Kremlin suspended a landmark nuclear arms control treaty over the West's support for Kyiv.
Hours before Biden spoke in Poland following a surprise visit to Ukraine, Russian President Vladimir Putin vowed that Moscow would achieve its objectives in Ukraine and accused the West of plotting to destroy Russia.
Alleging that the United States was turning the Ukraine war into a global conflict, Putin said Russia was suspending participation in the 2010 New START treaty, its last major arms control treaty with Washington.
Putin, upping the ante in what has become the biggest confrontation with the West since the 1962 Cuban Missile Crisis, also announced that new strategic systems had been put on combat duty and threatened to resume nuclear tests.
Biden proclaimed "unwavering" support for Kyiv and a commitment to bolstering NATO's eastern flank facing Russia, while rejecting Moscow's contention that the West was plotting to attack Russia.
"One year ago, the world was bracing for the fall of Kyiv," Biden said at Warsaw's Royal Castle. "I can report: Kyiv stands strong, Kyiv stands proud, it stands tall and, most important, it stands free.
"When President Putin ordered his tanks to roll into Ukraine, he thought we would roll over. He was wrong," he said.
U.S. Secretary of State Antony Blinken called Putin's suspension of its role in New START "deeply unfortunate and irresponsible". NATO Secretary-General Jens Stoltenberg said it made the world a more dangerous place, and urged Putin to reconsider.
Signed by then-U.S. President Barack Obama and his Russian counterpart Dmitry Medvedev in 2010, the treaty caps the number of strategic nuclear warheads that the countries can deploy.
Due to expire in 2026, it allows each country to physically check the other's nuclear arsenal, although tensions over Ukraine had already brought inspections to a halt.
The Russian leader said, without citing evidence, that some in Washington were considering breaking a moratorium on nuclear testing. " ... If the United States conducts tests, then we will. No one should have dangerous illusions that global strategic parity can be destroyed," Putin said.
"A week ago, I signed a decree on putting new ground-based strategic systems on combat duty."
It was not immediately clear which systems he meant.
Putin said Ukraine had sought to strike a facility deep inside Russia where it keeps nuclear bombers, a reference to the Engels air base. Ukraine has followed a policy of not publicly claiming responsibility for any attacks on Russian soil.
Putin, who has over the past year repeatedly hinted that Russia could use a nuclear weapon if threatened, was in effect saying that he could dismantle the architecture of nuclear arms control unless the West backs off in Ukraine.
Putin said the conflict had been forced on Russia, particularly by NATO's eastward expansion since the Cold War.
"The people of Ukraine have become the hostage of the Kyiv regime and its Western overlords, who have effectively occupied this country in the political, military and economic sense."
Kyiv and Western leaders such as Biden, who visited the Ukrainian capital on Monday, reject that narrative as an unfounded pretext for a land grab in a fellow former Soviet republic that Putin calls an artificial state, and say he must be made to lose his gamble on invasion.
Russia has suffered three major battlefield reverses in Ukraine but still control around a fifth of its neighbour. Tens of thousands of Ukrainian civilians and soldiers on both sides have been killed.
A senior aide to Ukraine's president said Putin's speech showed he had lost touch with reality.
"He is in a completely different reality, where there is no opportunity to conduct a dialogue about justice and international law," Mykhailo Podolyak, a political adviser to Ukrainian President Volodymyr Zelenskiy, told Reuters.
"Russia is at a dead end. In the most desperate situation. Everything that Russia will do next will only worsen its situation."
As Putin was speaking, at least one Russian rocket slammed into a busy street in the southern Ukrainian city of Kherson, killing six people.
Ukraine's military and city authorities said 12 others were wounded in the attack, which left a pool of blood on the pavement beside a mangled bus stop.
Local authorities said Kherson came under fire from multiple rocket launchers as Putin described the West as the aggressor in Ukraine and depicted Russia as not waging war on the Ukrainian people. Russia did not immediately comment on the incident.
Moscow has denied deliberately targeting civilians in its "special military operation", but cities across Ukraine have been devastated in missile and drone attacks and thousands of civilians have been killed.
The West has pledged tens of billions of dollars in military aid to Kyiv.
Speaking for an hour and 45 minutes, Putin vowed that Moscow would achieve its aims in Ukraine and thwart the U.S.-led NATO alliance in the process.
"They intend to transform a local conflict into a phase of global confrontation," he said. "This is exactly how we understand it all and we will react accordingly, because in this case we are talking about the existence of our country."
(Additional reporting by Alan Charlish, Pawel Florkiewicz, Anna Koper and Andrius Sytas in Warsaw, Rod Nickel in Kherson, Pavel Polityuk and Max Hunder in Kyiv, Steve Holland in Washington; Writing by Mark Heinrich; Editing by Alison Williams)