WASHINGTON — Pressure on Iran mounted on Thursday, with the United States saying it was determined to isolate the country’s central bank, and three of Iran’s largest oil customers — Japan, South Korea and China — getting assurances that Saudi Arabia and other Persian Gulf producers would help make up any gap in supplies if they curtailed oil purchases from Iran.

The Obama administration said its campaign to choke off Iran’s oil exports was making headway, amid signs that Japan, South Korea and even China were seeking alternatives to Iran, in order to comply with American sanctions on Iran’s central bank, through which most purchases are made.

“We do mean to close down the Central Bank of Iran,” said a senior administration official, adding that oil purchases were the key to that effort because oil “is the largest source of their revenue.”

Delegations from the State, Energy and Treasury Departments are fanning out to Iran’s major customers, as well as to rival oil producers, to enlist them in an ambitious project: to effectively cut off one of the world’s largest oil producers without driving up oil prices.