The ruble jumped 4.9 percent, to 55.8 per dollar, in Moscow on Monday after Hong Kong-based Phoenix TV cited China's Commerce Minister Gao Hucheng as saying that expanding the currency swap between the two nations would help Russia.

The ruble has gained 10 percent over the past two days, paring a selloff that's made it the world's worst performing currency over the past six months.

Unlike Ukraine, where the pro-West government received a $17 billion IMF-led bailout this year, Russia, Argentina, and Venezuela are often at odds with the U.S. and its allies, essentially keeping them out of the reach of the Washington-based institution. At $3.89 trillion, China holds the world's largest foreign-exchange reserves, allowing it to fill the void.

China and Russia signed a three-year currency-swap line of 150 billion yuan ($24 billion) in October, a contract that allows Russia to borrow the yuan and lend the ruble. While the offer won't relieve the main sources of pressure on the ruble—which has lost 41 percent this year amid plunging oil prices and sanctions linked to Russia's annexation of Crimea—it could bolster investors' confidence in the country and help stem capital outflows.

Argentina Reserves

Two phone calls to China's central bank seeking comment on the terms of its currency swaps weren't returned. Russia isn't in talks with China about any financial aid, Dmitry Peskov, a spokesman for President Vladimir Putin, said on Dec. 20.

Funding from China has helped raise Argentina's foreign reserves to a 13-month high of $30.9 billion, a boost for a country that has been kept out of international capital markets since defaulting on foreign obligations in 2001.

Argentina received $1 billion worth of yuan earlier this month as part of the three-year currency-swap agreement with China, a central bank official in the South American country, who asked not to be identified because he isn't authorized to speak publicly, said Dec. 11. That extended the funds transfered to Argentina to $2.3 billion since October. The swap is for a maximum of $11 billion over three years.

In Venezuela, President Nicolas Maduro last month added $4 billion he borrowed from China to the country's reserves after they fell to an 11-year low. The country now has about $21 billion in its coffers, equal to the amount of debt it has coming due in 2015 and 2016.

Venezuela, which was already plagued by shortages of everything from toilet paper to toothpaste, is also suffering from the drop in oil, its biggest export. Traders are betting that there's an 89 percent probability that Venezuela won't be able to make good on its debts over the next five years, according to credit-default swaps data compiled by Bloomberg.

“I don't think this is a broad policy to support any country that asks for Chinese help,” Steffen Reichold, an economist at Stone Harbor Investment Partners in New York, said in an email. “Several countries are currently in a tight spot and the Chinese are offering to help. That buys them some goodwill and influence, and promotes the use of the yuan.”

U.K., Australia

The People's Bank of China (PBOC) has signed currency-swap agreements with 28 other central banks around the world, including those in the U.K. and Australia, making the yuan an alternative to the dollar for global trade and finance.

By promoting the use of its currency, China acts in its own interests as it challenges the dominance of the U.S. in the global economy.

Two months after Russia annexed Crimea in March, China signed a three-decade, $400 billion deal to buy Russian gas. Oil imports from Russia hit an all-time high in November, according to China's General Administration of Customs. While the ruble's depreciation affected Chinese exports to Russia and made it difficult for the two countries to implement joint projects, the challenges shouldn't be exaggerated, according to a commentary published in the official People's Daily newspaper today.

China has made $47 billion in loans to Venezuela since 2007, making it the country's largest creditor, according to Eurasia Group, a political consulting firm. Venezuela, which holds the world's largest oil reserves, repays the loans by shipping crude to China.

In July, Xi signed trade and investment agreements for at least $7.5 billion in Argentina, cementing China's ties to the world's third largest soybean producer.

“China is playing an increasingly more important role and is willing to engage,” Michael Ganske, who oversees $8 billion as the head of emerging markets at Rogge Global Partners Plc, said by e-mail from London. “There is geo-strategic importance connected with” the funding deals, he said.

–With assistance from Katia Porzecanski in New York, Vernon Wessels in Johannesburg and Fion Li in Hong Kong.