United States President Joe Biden and Russian President Vladimir Putin bolstered their respective alliances with Nato and China, days before the first anniversary of Moscow's invasion of Ukraine.
Global tension between the superpowers was highlighted on a day of diplomacy in Warsaw and Moscow on Wednesday.
Biden promised that the US "will defend literally every inch of Nato ", the military alliance that includes some Eastern European countries bordering Russia.
Putin said Chinese leader Xi Jinping would visit Russia, saying relations had reached "new frontiers". Washington is concerned Beijing could provide material support for Moscow's war in Ukraine, which began on February 24 last year and has become the biggest land conflict in Europe since World War II.
The war has displaced millions, left Ukrainian cities, towns and villages in ruins and disrupted the global economy.
Putin, in an address to mark a “Defender of the Fatherland” public holiday on Thursday, said Russia would continue to pay increased attention to boosting its nuclear forces and will begin mass deliveries of Zirkon sea-launched hypersonic missiles.
Russia is participating in a 10-day joint naval exercise with South Africa and China, which is taking place off South Africa's eastern coast and coincides with the one-year anniversary of Russia's invasion of Ukraine.
Moscow sent a frigate equipped with hypersonic missiles. Russia's TASS news agency reported earlier this month that a frigate participating in the drills would perform a training launch of the Zircon missile system during the exercise.
However, Captain Oleg Gladkiy – who is heading the Russian contingent – said during a media briefing on Wednesday that Russia would not fire its new generation "Zircon" hypersonic missile during the exercise.
"The hypersonic weapon will not be used in the context of these exercises," Gladkiy said. "There is no hidden meaning in the exercises that we are performing today."
Putin has called the Zircon, which can travel at more than five times the speed of sound, "unstoppable".
In remarks issued by the Kremlin on Thursday, Putin said Russia would also keep equipping its armed forces with advanced equipment and is boosting the production of conventional weapons.
On Wednesday, Putin welcomed China's top diplomat, Wang Yi,to the Kremlin. Wang also met separately with Russian Foreign Minister Sergey Lavrov.
TASS news agency quoted Wang as saying China would "firmly adhere to an objective and impartial position and play a constructive role in the political settlement of the crisis".
Lavrov and Wang had not discussed a reported Chinese peace plan, the Russian foreign ministry said.
The relationship between China and Russia, Wang said through an interpreter, was not directed against any third party, but in a jab at the US he said the countries would "not succumb to pressure from third parties".
Eastern flank of Nato
In Warsaw, Biden met with leaders of nine Eastern European countries that during the Cold War were either aligned with Moscow or part of the former Soviet Union. Some of them now are among the strongest supporters of military aid to Ukraine.
"As Nato's eastern flank, you are the front line of our collective defence. You know better than anyone what is at stake in this conflict. Not just for Ukraine, but for the freedom of democracies throughout Europe and around the world," Biden said.
The Kremlin says it regards Nato, which could soon expand to include Sweden and Finland, as an existential threat to Russia.
On his trip this week, which included a visit to the Ukrainian capital Kyiv, Biden rejected Russia's contention that the West wants to control or destroy it.
Meanwhile, at a meeting on Wednesday of the 193-member United Nations General Assembly to mark the Ukraine war's anniversary, UN Secretary-General António Guterres denounced Russia's invasion as a violation of the founding UN Charter and international law and called out Moscow's threats about the possible use of nuclear weapons.
In two speeches last September, Putin indicated that he would, if needed, use nuclear weapons to defend Russia.
"We have heard implicit threats to use nuclear weapons. The so-called tactical use of nuclear weapons is utterly unacceptable. It is high time to step back from the brink," Guterres said.
Putin suspended the New START (Strategic Arms Reduction Treaty) on nuclear arms control with the US on Tuesday, accusing Washington of turning the war into a global conflict by arming Ukraine.
Moscow would stick to agreed limits on nuclear missiles and keep informing the US about changes in its deployments, a senior Russian defence ministry official said.
Biden said, in an ABC News interview, that he saw no change in Moscow's nuclear posture and did not think Putin's remarks meant Russia was thinking of using nuclear weapons.
Asked about Russia's suspension of participation in New START, Biden called it "a big mistake" but said, "I don't read into that that he's thinking of using nuclear weapons or anything like that".
Moscow has launched repeated assaults to secure full control of two eastern provinces forming Ukraine's Donbas industrial region. Russian troops managed to break through Ukrainian defences near the town of Kreminna on Tuesday but were pushed back and lost some of their heavy equipment, a senior Ukrainian official said on Wednesday.
Kreminna is about 70km north of Bakhmut, a focus of the fiercest Russian attacks.
Russia controls around a fifth of Ukraine a year after starting what it called a "special military operation" to protect Russian security.
Ukraine and its Western allies have described the invasion as an imperial-style land grab.