U.S. Treasury Secretary Timothy Geithner’s efforts to tighten economic sanctions on Iran over its nuclear program won backing from Japan a day after China rejected limiting oil imports from the country. (Kiyoshi Ota/BLOOMBERG)

Japan agreed Thursday to cut its dependence on Iranian oil, bolstering the U.S.-led effort to choke off funding for Iran’s nuclear development program.

The Japanese pledge, made during a visit here by Treasury Secretary Timothy F. Geithner, stood in contrast to resistance this week from China, where officials say that Iran’s uranium program and its energy sales should not be linked.

Japan imports roughly 10 percent of its oil from Iran, making it the Islamic republic’s second-biggest oil customer, behind China. Japanese Finance Minister Jun Azumi said Thursday that Japan would cut its use of Iranian oil “as soon as possible in a planned manner.”

Japan has already reduced its reliance on Iranian oil by 40 percent in the past five years, Azumi said.

Geithner undertook his quick trip to Asia’s two largest economies as part of the Obama administration’s effort to tighten sanctions on Iran, taking primary aim at its oil industry.

Japanese Prime Minister Yoshihiko Noda told Geithner that he shared Washington’s concern about Iran’s nuclear program. But he also expressed worry about the targeting of Iran’s oil sector, saying that the sanctions “could cause serious effects on the Japanese and world economies,” according to a government statement.

Oil prices are unlikely to rise sharply as long as China stands aloof from the U.S. campaign, analysts say. But if the sanctions take effect as envisaged, Iran could lose its major customers, all but eliminating it as an oil exporter. Oil prices would jump, and countries such as Japan — which is only 16 percent energy self-sufficient, with very few natural resources of its own — would face an increased burden.

Japan imports more than three-quarters of its oil from the Middle East, mostly Saudi Arabia and the United Arab Emirates.

Since last year’s earthquake and tsunami triggered meltdowns at three nuclear reactors, the country has turned away from nuclear power. Only six of its 54 reactors are online.

Separately Thursday, the Obama administration announced sanctions against three foreign companies — one of them from China — for trading with Iranian energy firms. The firms will be barred from obtaining U.S. export licenses or financing because of their history of allegedly selling gasoline or other refined petroleum products to Iran, the State Department said in a statement.

The three firms were identified as Zhuhai Zhenrong of China, Kuo Oil of Singapore and FAL Oil of the United Arab Emirates. U.S. officials said the companies had supplied Iran with more than $600 million worth of petroleum products in the past two years.

Staff writer Joby Warrick in Washington contributed to this report.