Chinese and Indian refiners are seeking alternative supplies of crude as new U.S. sanctions on Russian producers and tankers are set to be the most effective yet in curbing shipments to Moscow's biggest customers, numerous traders said on Monday.

The U.S. Treasury on Friday imposed sanctions on Russian oil producers Gazprom Neft and Surgutneftegaz, as well as on 183 vessels that have shipped Russian oil, as it targets the revenues Moscow has used to fund its war with Ukraine.

Many of the tankers have been used to ship oil to India and China as Western sanctions and a price cap imposed by the Group of Seven countries in 2022 shifted trade in Russian oil from Europe to Asia. In addition, some tankers have shipped oil from Iran, which is also under sanctions.

On Monday, China reiterated its opposition to unilateral U.S. sanctions.

Whereas Chinese and Indian refiners have adapted to previous sanctions, the severity of the new measures has driven them back to sellers of oil that is not restricted, curbing supply and driving up spot premiums for crude produced in the Middle East to Africa and Brazil.

Global Brent crude futures have also risen. On Monday they climbed above $81 a barrel to their highest since August.

In an early illustration of the impact on shipping activity, five tankers under sanctions have been anchored off Shandong province since Friday, shipping data on LSEG Workspace showed. Another is on the way.

Traders said the ships are not allowed to discharge oil after Shandong Port Group banned tankers under U.S. sanctions from calling at its ports.

Over the weekend, new Chinese refiner Yulong Petrochemical bought 4 million barrels of Abu Dhabi's Upper Zakum crude loading in February and March from Totsa, the trading arm of French energy major TotalEnergies, traders said.

The cargoes are for its 400,000 barrel per day (bpd) refining complex in Yantai, eastern Shandong province, which started trial runs in September.

Yulong, which has previously bought Russian ESPO Blend crude, has purchased Angolan and Brazilian crude in recent weeks, traders said, and is in talks to buy more oil from West Africa as well as Canada.

The refiner purchased 2 million barrels of Angolan Girassol and Nemba crude and also 2 million barrels of Brazilian Buzios and Tupi crude, they said.

The numerous sources Reuters spoke to declined to be named as they were not authorised to speak to media. Yulong and Totsa typically do not comment on commercial deals.

Indian refiners, which bought spot Middle East crude last week before the sanctions were announced, are looking for more cargoes, traders said.

India's Bharat Petroleum Corp Ltd bought 2 million barrels of February-loading Oman crude from Totsa via a tender last week, two people familiar with the matter said.

India will allow Russian oil cargoes booked before Jan. 10 to discharge at ports, the source told reporters, adding that supply will continue to flow during a sanctions waiver in place until March.

The source said Russia may deepen discounts for crude exports to India to comply with the $60 a barrel price cap, allowing them to continue.

The strong demand is helping Totsa to reduce an overhang of Middle East crude supplies after it amassed cargoes via the S&P Global Platts' trading platform over the last four months, traders said.

Spot premiums for Middle East benchmark grades jumped more than 70% to about $3 a barrel on Monday, traders said, reaching their highest since October 2023.

The premiums for sweet grades have also risen, with Brazilian crude for March delivery transacting at premiums of more than $3 a barrel to dated Brent last week, up about $2 from levels seen in early December, one of the traders said.

A trading executive involved in the Russian oil business said the biggest disruption would be to shipping, and that complications could arise if a ship is owned or managed by companies involved in operating tankers under sanctions.

Over time, the market is likely to see a growing number of middlemen marketing oil from Gazprom Neft and Surgutneftegaz, and there will be more payments in Chinese yuan via China's Cross-border Interbank Payment System (CIPS), the executive said.

Also included on Friday's sanction document were two Chinese oil logistics firms - Shandong United Energy Pipeline Transportation Co Ltd and Guangrao Lianhe Energy Pipeline Conveyor Co - both based in eastern China's Shandong province, a refining hub and China's main destination for oil under sanctions.

As these companies mostly transport oil from storage tanks to domestic refiners with payments in Chinese yuan, there would be little impact from the sanctions, the trading executive added.

(Reporting by Florence Tan, Siyi Liu and Chen Aizhu in Singapore, and Nidhi Verma in New Delhi; Additional reporting by Trixie Yap in Singapore and Ethan Wang in Beijing; Editing by Kate Mayberry and Barbara Lewis)