BEIJING/MOSCOW (Reuters) -Chinese President Xi Jinping pledged to work with counterpart Vladimir Putin to "rejuvenate" their countries as the pair started a day of talks in Beijing, saying China would "always be a good partner" of Russia, according to Chinese state media.

Putin arrived earlier on Thursday for a two-day state visit that will include detailed talks on Ukraine, Asia, energy and trade with Xi, his most powerful political backer and fellow geopolitical rival of the United States.

"The China-Russia relationship today is hard-earned, and the two sides need to cherish and nurture it," Xi told Putin as they met at the Great Hall of the People in Beijing for their opening session.

"China is willing to...jointly achieve the development and rejuvenation of our respective countries, and work together to uphold fairness and justice in the world."

China and Russia declared a "no limits" partnership in February 2022 when Putin visited Beijing just days before he sent tens of thousands of troops into Ukraine, triggering the deadliest land war in Europe since World War Two.

By picking China for his first foreign trip since being sworn in for a six-year term that will keep him in power until at least 2030, Putin is sending a message to the world about his priorities and the strength of his personal ties with Xi.

As they met, Putin told Xi that their co-operation was a stabilising factor.

"It is of crucial significance that relations between Russia and China are not opportunistic and are not directed against anyone," Russia's RIA Novosti news agency cited Putin as saying.

In an earlier interview with China's Xinhua news agency before his departure, Putin praised Xi for helping to build a "strategic partnership" with Russia based on national interests and deep mutual trust.

"It was the unprecedentedly high level of the strategic partnership between our countries that determined my choice of China as the first state that I would visit after taking office as president," Putin said.

"We will try to establish closer co-operation in the fields of industry and high technology, space and peaceful nuclear energy, artificial intelligence, renewable energy sources and other innovative sectors."

Informal chats between the leaders and senior officials of both sides to be held over tea and dinner later on Thursday are expected to be key to the two-day trip.

Kremlin foreign policy adviser Yuri Ushakov said those talks would range over Ukraine, Asia, energy and trade.

Putin's newly appointed defence minister, Andrei Belousov, as well as Foreign Minister Sergei Lavrov, Security Council Secretary Sergei Shoigu and Ushakov will also attend, along with Russia's most powerful CEOs.

It was not immediately clear if Gazprom CEO Alexei Miller would go to China as he was on a working visit to Iran on Wednesday.

CELEBRATION OF 75 YEARS OF TIES

Putin, 71, and Xi, 70, will participate in a gala celebration of 75 years since the Soviet Union recognised the People's Republic of China, which Mao Zedong declared in 1949.

Xinhua confirmed Putin's arrival for a state visit and the expected talks with Xi, while dozens of large Russian and Chinese flags fluttered around Tiananmen Square amid police patrols.

Some commentaries have hailed the pair's "great power diplomacy".

The event is the top trending item on the Chinese social media platform Weibo, with 1.4 million search requests.

The United States casts China as its biggest competitor and Russia as its biggest nation-state threat while U.S. President Joe Biden says this century will be defined by an existential contest between democracies and autocracies.

Putin and Xi share a broad world view, which casts the West as decadent and declining, just as China challenges U.S. supremacy in everything from quantum computing and synthetic biology to espionage and hard military power.

Putin will also visit the northeastern city of Harbin, which has historic ties to Russia. A mall devoted to Russian-made goods from about 80 Russian manufacturers opened on Thursday, the China Daily said.

China has strengthened trade and military ties with Russia in recent years as the United States and its allies imposed sanctions on both countries, particularly Moscow, for its invasion of Ukraine.

Western governments say China has played a crucial role in helping Russia withstand the sanctions and has supplied key technology that Russia has used on the battlefield in Ukraine.

But China, once Moscow's junior partner in the global Communist hierarchy, is by far the most powerful of Russia's friends globally.

Putin's arrival follows a mission to Beijing late last month by U.S. Secretary of State Antony Blinken, in part to warn China's top diplomat Wang Yi against deepening military support for Russia.