The squeeze is already beginning on Iran's oil exports -- and guess which nation quietly reduced its purchases from Tehran this month? Why, that would be China, Iran's supposed protector.

The Chinese cut their imports from Iran roughly in half for January, trimming 285,000 barrels per day from their average last year of about 550,000 barrels per day, according to Nat Kern, the publisher of Foreign Reports, a respected industry newsletter.

Iran's reduced sales to China, its biggest oil customer, resulted from a dispute over payment terms, Kern explains. But it's an early sign of what may be significant reductions in Iranian exports to Europe and Asia, as buyers there hedge against the likelihood of tighter sanctions.

Here's how Kern and other oil industry analysts add up the potential dents in Iran's exports, which were 2.2 million barrels a day last year. First, U.S. allies are considering sanctions: Europe has agreed on an embargo that by year-end could cut about 450,000 daily barrels; Japan is talking about cutting 100,000 barrels; South Korean officials have discussed a reduction of 40,000 barrels. Even non-aligned countries are getting nervous about Iran's reliability: Indian refineries, for example, bought extra oil from Saudi Arabia in January "just in case."

Analysts reckon that even if sanctions are only partly successful, Iran is likely to lose about 20 percent of its oil export volume and 25 percent of its revenues. For an economy that is already weak, that loss of revenue will be painful.

The payment haggle between Iran and China illustrates Tehran's new vulnerability. According to Kern, the Chinese cut the 285,000 barrels when Iran refused a request for better credit terms. The difference amounted to just 50 cents a barrel, but the Iranians apparently feared that if they gave China a discount, other purchasers would want one, too.

As China puts the screws to Iran, it's warming its relations with Saudi Arabia -- which can cushion the market by tapping its 2 million barrels a day of spare capacity. A symbol of closer Sino-Saudi contact was the visit to Riyadh last weekend by Prime Minister Wen Jiabao, who inked a $10 billion joint refinery project.

Kern says Iran probably won't be able to restore the lost exports to China until March or April. "At that time, the Obama administration could point to a substantial reduction in China's imports of Iranian crude as a basis for excepting Chinese banks from the punitive measures," explains Kern's current newsletter.

Can the U.S.-led effort cripple Iran's oil exports without triggering a panicky price rise? The problem is eased in part by Saudi Arabia's extra capacity. Saudi officials have said they aren't seeking to replace Iranian oil, but simply to supply what the market needs -- still, the effect is the same. Meanwhile, increased production from Libya and Iraq will likely add another 500,000 barrels a day. All told, experts reckon there's a buffer of about 3 million barrels a day -- making for a thin cushion of spare capacity if all 2.2 million barrels of Iran's exports were somehow curtailed.

And what will Tehran do, if Europeans and Asians reduce purchases of Iranian oil? Probably load it in tankers and park them outside the Strait of Hormuz. The Iranians might imagine they'd be sitting pretty if they tried to close the strait -- but experts note their floating reserve would also be hard to protect, and vulnerable to seizure.

And all that extra oil afloat might have a downward effect on prices, notes Kern. Almost any way you look at it, Iran is likely to have an oil problem in 2012.