NEW YORK – Aluminium prices are heading for the best week in at least three decades as US sanctions on Russian producer United Co. Rusal send buyers scrambling to find supplies.

Prices of the metal climbed to the highest since 2012 Thursday, helping push this week’s gain to almost 14%, the most in Bloomberg records going back to June 1987. The US premium, or the amount added to the London Metal Exchange price of aluminium to ship metal to the Midwest, jumped by the most on record.

On Thursday, Rusal was said to declare force majeure on some shipments of alumina, used to make aluminium. That comes a day after Glencore, the biggest buyer of metal from Rusal, was said to have invoked the same legal clause on aluminium. The moves compound supply worries after the LME and CME Group’s Comex said they won’t allow new deliveries of metal from Russian billionaire Oleg Deripaska’s Rusal, the biggest aluminium producer outside China.

“People are rushing to secure supply,” Ryan McKay, a commodity strategist at TD Securities in Toronto, said in a telephone interview. “People are expecting that a significant amount of the metal produced by Rusal is going to be off the market.”

Aluminium for delivery in three months climbed as much as 3.6% to $2 331 a metric ton on the LME, the highest since March 2012. Prices settled 3.3% higher at $2 325 a ton at 5:51 pm in London.

Alcoa, the biggest US aluminium producer, advanced 0.9% to $54.47 on Thursday in New York. The Pittsburgh-based company has jumped 13% this week, on course for the biggest weekly gain of 2018.

The surge in the aluminium premium that buyers pay to have the LME metal delivered to the Midwest shows consumers in the US are already feeling the pinch.

The duty-paid premium surged to a range of 21.75 cents to 23.55 cents a pound, according to Austin, Texas-based researcher Harbor Intelligence, up from a prior range of 17.25 cents to 18 cents.

The “increase has no precedent,” Jorge Vazquez, Harbor’s managing director, said in report dated Wednesday.

Force majeure is a legal clause that allows trader to leave contracts unfulfilled because of circumstances beyond their control.