Offers of Iranian crude to Chinese buyers have declined and prices have jumped this week as the US blockade has cut Tehran’s shipments, Reuters reports, citing trade sources, with the threat of more sanctions from Washington looming.

The number of offers for Iranian oil cargoes to China for September and October delivery has declined from July and August cargoes, four trade sources familiar with the matter said. The offers have declined as barrels already in ships on the water have been sold, they said.

Iran’s oil exports have fallen since mid-July, with no visible crossings of the Strait of Hormuz by supertankers carrying Iranian crude since then.

The squeeze threatens a key feedstock for independent refiners, known colloquially as teapots, located in China’s eastern province of Shandong, which account for about a fifth of China’s refining capacity and are the top buyers of sanctioned oil.

Three of the trade sources said some Iranian crude, typically sold at discounts, was being offered at premiums to ICE Brent futures, with one source citing a premium of about $2 a barrel. That was an abrupt shift as cargoes of Iranian Light were being offered earlier this week at a discount of around $3 a barrel, the same as a month earlier.