The linchpin of the relationship between Mr. Xi and Mr. Putin was a May 2014 accord on a 30-year deal for China to buy natural gas from fields in Eastern Siberia, for a reported $400 billion, with first delivery between 2019 and 2021. During the signing in Shanghai, Mr. Putin bragged that the deal was an “epochal event” and expressed relief that Russia, under pressure from European sanctions, would be able to diversify its gas sales.

But the price was never formally announced, and it is possible that with plunging energy prices, the deal will have to be renegotiated, said Jonathan Stern, chairman of the natural gas research program at the Oxford Institute for Energy Studies in Britain. The Chinese wanted the gas for its depressed northeast region, and the Russians had started to prepare for its delivery, but there has been only limited drilling, he said.

Another deal, for natural gas from Western Siberia, was initialed by the two leaders in November in Beijing, but a formal contract that was expected to be signed in Beijing during Mr. Putin’s current visit appears to have fallen by the wayside, Professor Stern said. “This is the contract which Putin could have signed this week, but we understand will not, partly because Chinese gas demand now looks much lower than previously thought,” he said.

Further complicating that deal is Russia’s inability to pay for the pipelines, and the question of whether China needs the Russian gas badly enough to finance their construction, said Edward C. Chow, senior fellow at the Center for Strategic and International Studies in Washington. “China will have to pay for the construction, one way or another, given Russia’s financial crunch,” he said.

A Chinese expert on Russia, who is usually sanguine about the relationship with Moscow, said that the deal had also run into pricing problems. “The negotiations face many difficulties due to the plunge in the price of gas,” said Zhao Huasheng, director of the Center for Russia and Central Asia Studies at Fudan University in Shanghai. “We have to recalculate all the costs and try to push for a price cut.”