Under pressure from the US, some of the top Asian economies are reluctantly looking for options to reduce the amount of oil they buy from Iran, an action that could further tighten the economic vise on an increasingly defiant nation that announced plans for a new round of naval drills in the Strait of Hormuz.

The decision by South Korea and Japan to try to accommodate Washington's demands followed reports that China had already reduced its purchase of Iranian crude in the past month in a pricing dispute with Tehran. Whatever the motives, the combined loss of sales threatens an Iranian economy already reeling, where the currency has plummeted in value, inflation has surged and the general public has expressed growing anxiety about the prospect of war.

China, Japan, India and South Korea together import more than 60 per cent of Iranian oil exports, and they all depend on Iran and other Gulf producers for the preponderance of their oil and natural gas needs. As tensions in the Gulf have increased and alarmed Asian governments and businesses, companies and traders from those countries have been putting out feelers to places like Russia, Vietnam, West Africa, Iraq and especially Saudi Arabia to export more oil to them, according to oil experts.

For Tehran, which relies heavily on oil revenues to p­rop up an economy batt­e­r­ed by years of sanctions, the potential cutbacks by Asian customers follow a decision by the European Union to move toward a ban on the import of Iranian oil.

Taken together, the Western efforts represent the most serious economic pressure yet on Iran after years of conflict over a nuclear programme that the West charges is aimed at building weapons.

But if the goal is to force Iran to relent, the campaign has so far had an opposite effect: Iranian officials have equated the focus on their oil market with economic war and threatened to block the Strait of Hormuz, where about one-fifth of the world’s oil passes to get to market.

The Iranian military, fresh from 10 days of naval exercises near the strait that ended last week, vowed to hold a new round of war games soon. The defense minister, Brig Gen Ahmad Vahidi, in comments reported by the semiofficial Fars News Agency on Thursday, said the military’s exercises would be "its greatest naval war games" and would occur "in the same region in the near future."

The US, however, is keeping the pressure on.

Treasury secretary Timothy F Geithner is scheduled to visit Beijing and Tokyo this week, and the tightening sanctions on Iran will be high on his agenda. US officials said Geithner would press China to reduce its purchases of Iranian oil. The trip is part of a concerted effort by Western diplomats to persuade Asian countries to go along with new European sanctions, US and European officials say.

“It’s a global chess game," said Daniel Yergin, an energy historian. "The major buyers are prudently beginning to make alternative plans to reduce their reliance on Iranian oil.”

The Asian efforts to wean themselves from Iranian crude are in response to legislation enacted by the US Congress last month requiring the administration to phase in sanctions in stages by late June that would make it very difficult for others to buy Iranian oil, by barring transactions with Iran’s central bank.

The sanctions exempt food, medicine and other humanitarian trade and include a presidential waiver for any country or company in cases in which the effects would harm the national security interests of the US.

More significantly, the legislation exempts from sanctions countries that "significantly reduce" imports from Iran. The legislation does not define this term, leaving it to the administration, according to officials.