MOSCOW (NYTIMES)- Russia is increasingly wielding oil as a geopolitical tool, spreading its influence around the world and challenging the interests of the United States.

But Moscow risks running into trouble, as it lends money and makes deals in turbulent economies and shaky political climates.

The strategy faces a crucial test this week in Venezuela, a Russian ally that must come up with US$1 billion (S$1.36 billion) to avert defaults on its debts.

Russia has been making a flurry of loans and deals all centred on the Venezuelan oil business, money that could make the difference between the government's collapse and its survival. In return, Moscow is getting a strategic advantage in Washington's backyard.

President Nicolás Maduro of Venezuela was all smiles this month on a visit to Moscow seeking fresh financial backing, thanking President Vladimir Putin of Russia "for your support, both political and diplomatic."

Moscow, through the state oil giant Rosneft, is trying to build influence in places where the United States has stumbled or power is up for grabs. Its efforts are also driven out of necessity, as US and European sanctions have forced Rosneft to find new partners and investments elsewhere.

The company, which Russia has long relied on to finance its government and social programs, has been pushing deeply into politically sensitive countries like Cuba, China, Egypt and Vietnam, as well as tumultuous places where US interests are at stake.

Rosneft is looking for deals around the eastern Mediterranean and Africa, areas of tactical importance beyond the energy picture. It is wielding economic and political sway in northern Iraq, by making big oil and natural-gas deals in Kurdish territory. And it is angling to bid for control of Iranian oil fields as tensions between Teheran and Washington escalate.

Rosneft is "trying to create opportunities that can be extremely valuable in geopolitical ways," said Amy Myers Jaffe, an energy security expert at the Council on Foreign Relations. "They really give the Russian government unbelievable leverage on questions of importance to the United States."

The new push by Rosneft follows a clampdown on Russia.

Rosneft, which is 50 per cent owned by the Russian state, is led by Igor Sechin, a former deputy prime minister and a close Putin ally. After the Russian invasion of Crimea three years ago, the United States and Europe hit Sechin with sanctions.

Since then, Exxon Mobil and other Western oil companies have been prevented from using their technological expertise to help Rosneft develop deepwater, shale and Arctic oil and gas fields. That has forced Rosneft to go far and wide to find new oil fields to replace its reserves.

Rosneft's biggest bet so far is Venezuela. Over the past three years, Russia and Rosneft have provided Caracas with US$10 billion in financial assistance, helping Venezuela stave off default at least twice under the weight of as much as US$150 billion in debt.

Russia is effectively taking China's place as Venezuela's principal banker. While President Hugo Chávez was in power, China lent Venezuela tens of billions of dollars for projects to be paid for with oil. But China quietly stopped making new loans, leaving Russia to fill the void.

Last year, Rosneft took a 49.9 per cent stake in Citgo, the Venezuelan state oil company's refining subsidiary in the United States, as collateral for a US$1.5 billion loan to the Venezuelan company. The state oil company, Petróleos de Venezuela, or PDVSA, used the money to pay its bills and keep its oil fields producing.

The deal was sharply criticised by members of Congress, who warned that an eventual Russian takeover of Citgo would threaten national security. Citgo operates about 4 per cent of US refining capacity and has a sprawling network of pipelines and gas stations. And Caracas remains highly dependent on the US market, since few refineries outside the United States can process large quantities of low-quality Venezuela crude.

In April, Rosneft went further, providing a US$1 billion advance payment for crude oil produced by the state oil company, crucial aid for it to make nearly US$3 billion in payments to bondholders.

But Russia's investments are not without risk. Venezuela's oil fields are aging and in disrepair. Oil service companies have been withdrawing after years of partial payments for their work. And fresh US sanctions have largely prohibited long-term loan transactions with PDVSA or investments in other new government debt, making Venezuela's financial straits even more acute.

"Russia is the only country that can give Venezuela a lifeline to survive through the rest of the year," said Francisco J. Monaldi, an energy policy analyst at Rice University. "China has the capacity but not the willingness to do it, and that's why Venezuela is so desperate to get the Russian support. There is no other way out."

Venezuela is now Rosneft's second-largest source of crude, after Russia itself. The Russian company resells about 225,000 barrels a day of Venezuelan oil, equivalent to 13 per cent of Venezuela's exports.

All the wheeling and dealing appears to be at its most aggressive in Venezuela, where Moscow's engagement is more risky.

The Venezuelan government says it has more than US$9 billion in currency reserves, though much of that is gold that must be sent abroad to liquidate, a transaction that can take time.

The next major debt payment comes due Thursday, for US$1.2 billion, on a PDVSA bond that is maturing. Flirting with default, the company scrambled to pay most but not all of a US$1 billion bond due Friday, while the country still owes US$350 million more in payments that were due this month.