LONDON (Reuters) - Swiss-based trading house Glencore and aluminium producer Rusal have extended their long-term supply contract into next year after the amount traded so far has amounted to a fraction of the agreement's maximum volumes, a source with direct knowledge said on Wednesday.

Glencore's contract with Russia's Rusal, the world's largest aluminium producer outside China, was due to expire this year unless extended.

Rusal and Glencore declined to comment.

Glencore is legally bound to continue the contract because Rusal has not been directly targeted by Western sanctions, the source, who declined to be identified, told Reuters.

Last month, the London Metal Exchange (LME) broadly banned Russian aluminium, copper and nickel produced from April 13 to comply with new U.S. and UK sanctions imposed over Russia's 2022 invasion of Ukraine.

During the first four years of the contract up to last year, Glencore bought about one-third of the maximum 5.24 million metric tons, calculations by Reuters based on regulatory filings show.

"The remaining volumes will be taken at a later date," the source said.

In 2023, Glencore bought aluminium worth $1.06 billion under the contract compared to a maximum of $3.93 billion worth, Rusal said in its annual report last month.

Last year's purchases by Glencore from Rusal would amount to around 459,000 tons, according to a Reuters calculation based on the average LME cash price of $2,307 per ton for 2023.

It is a rough estimate because the price would also include regional premiums that depend on the shipments' destinations. Rusal's total 2023 aluminium sales were 4.2 million tons.

(Reporting by Eric Onstad and Polina Devitt; editing by Rod Nickel)

By Eric Onstad and Polina Devitt