唐寧街說，普京總統動員俄羅斯的預備役軍人是 "明確承認 "他對烏克蘭的入侵正在失敗。
普京先生指責西方想要摧毀俄羅斯，並強調他將使用 "所有可用的手段 "來保護其領土。
Downing Street says President Putin's mobilisation of Russia's military reservists is a "clear admission" his invasion of Ukraine is failing.
No 10 condemned Moscow's "reprehensible actions" and said the UK would continue to help Ukraine defend its sovereignty.
The prime minister will address the United Nations General Assembly in New York early on Thursday morning.
Downing Street said Liz Truss would warn her fellow democratic leaders against any complacency when it came to defending their values and preserving a world order that rewards freedom.
She will also call on democracies "to harness the power of cooperation seen since Putin's invasion of Ukraine to constrain authoritarianism", it said.
A No 10 spokeswoman said: "Putin's speech...and his move to mobilise the Russian population are a clear admission that his invasion is failing.
Mr Putin accused the West of wanting to destroy Russia and stressed he would use "all available means" to protect its territory.
Meanwhile, Western nations have condemned Moscow's plans to hold so-called referendums in parts of Ukraine that are currently under Russian control.