Russian President Vladimir Putin and Chinese President Jinping signed agreements to extend their strategic partnership up to 2030 during talks in Moscow dominated by energy and trade issues.
Beijing's recent peace initiative for Ukraine, which called for a ceasefire but did not make concrete demands for the withdrawal of Russian forces, was also warmly received by Putin.
Xi's visit was denounced in Washington, with National Security Council spokesman John Kirby saying relations between Beijing and Moscow represent a "marriage of convenience" rather than a real alliance.
"If China wants to play a constructive role here in this conflict, then they ought to press Russia to pull its troops out of Ukraine and Ukrainian sovereign territory," Kirby said, adding that Xi needed to speak to Ukrainian President Volodymyr Zelensky.
At a news conference, Xi said he had held "constructive talks" at the Kremlin on the second of his three-day state visit, pointing in particular to the expansion of economic cooperation with Russia.
Putin assured Xi of a reliable supply of Russian oil and gas in the long term and said a new Russia-China gas pipeline via Mongolia is in the works.
Russia has been shut out of much of the European energy market after the invasion of Ukraine a year ago. Since then, Russia has sought out new customers and emphasized opportunities in Asia.
By 2030, gas supplies to China should rise to almost 100 billion cubic metres per year, Putin said.
In addition, 100 million tonnes of liquefied natural gas would be supplied, as well as coal and other energy sources. China will receive the energy at a discount.
Russia was ready to supply agricultural products to China, too, Putin said.
Payments for goods in the Chinese currency yuan and in roubles are also to be expanded, Putin said, while the two countries also plan to expand their transport links by building roads and bridges.
Putin called the talks "warm and collegial."
Turning to Russia's war on Ukraine, Putin again praised Xi's proposal for peace, which has been met with deep scepticism in Washington and Europe.
"We find that many of the positions in the peace plan put forward by China agree with Russian approaches and could become the basis for a peaceful solution, once the West and Kiev are ready for it," Putin said.
Xi said that China was taking an "objective and impartial position" on the conflict.
Kremlin spokesman Dmitri Peskov said the two had discussed Ukraine for more than four hours on Monday. "There was an opportunity to clear everything up," Peskov said.
For international observers, however, China is by no means a neutral authority - especially because the country, which is allied with Russia, has never condemned Russia's full-scale invasion of Ukraine.
Nothing was disclosed about possible arms and ammunition deliveries from China to Russia - a move that Washington and NATO said Beijing is considering.
In Brussels, NATO Secretary General Jens Stoltenberg warned China against supplying Russia with weapons as this "would be to support an illegal war."
NATO has not seen "any proof" that China is delivering weapons but the alliance has "seen some signs that his has been a request from Russia," Stoltenberg said.
The top NATO official said providing arms "is an issue that is [being] considered in Beijing" and urged China not to do so.
Xi arrived in Moscow on Monday and was expected to stay until Wednesday.
Xi's visit comes at an opportune moment for Putin. The visit is the first by a foreign leader since the International Criminal Court (ICC) issued an arrest warrant on Putin for alleged war crimes in Ukraine. He is accused of unlawfully deporting children from Ukraine to Russia. Russia says the children were being taken out of a war zone to safety.
For Putin, the visit from Beijing allows him to show that he is not isolated internationally.
According to Russian analysts, Xi fulfilled his most important mission immediately upon arrival in Moscow. His demonstrative support signalled to the West that Putin was here to stay, said political scientist Sergei Malakhov.
"China has effectively wiped away the question of international isolation," Malakhov told Russia's Vedomosti newspaper.
Xi said that it was in line with "historical logic" that he chose Russia for his first visit after his re-election, because both large countries are neighbours and "strategic partners."
The two allies issued a joint statement on international issues, showing where their interests align. They called for an objective investigation into the Nord Stream 1 and 2 explosions and spoke out against US dominance and in favour of a multipolar world order.
They also stressed that their strategic partnership was not a military-political bloc and not directed against other states.
Although China is economically benefiting from its partnership with Russia, Beijing is careful not to blatantly violate Western sanctions.
China is striking a careful balance of assuring stability and political support from its neighbouring country that it shares a 4,000-kilometre-long border with, while still leaving the door open for Europe, which is ultimately an even more important trade partner than Russia.
