China's crude oil imports from Iran plunged last month to their lowest since March 2012 when differences over payment terms of annual contract deals slashed shipments from Iran nearly in half.

The fall in purchases by Iran's top customer in January helps back the International Energy Agency's (IEA) estimate that the Opec-member's oil output likely fell further from its lowest in three decades due to tighter Western sanctions. The tough measures have made it difficult for buyers to pay Iran for its oil and find tankers to ship it, hurting Tehran's revenues.

The IEA, the West's energy agency, said preliminary data suggested Iran's oil exports could have fallen below 1mn bpd in January.

China, the world's second-largest oil consumer, bought 1.316mn tonnes of Iranian crude in January, or about 309,906 bpd, versus 593,400 bpd in December, according to data from the General Administration of Customs.

China imported 490,727 bpd of Iranian crude in January 2012, and shipments in March plunged to 253,300 bpd before the two sides settled the contractual differences for last year's oil.

Analysts were surprised at the extent of January's decline after December's crude imports jumped to last year's second highest daily levels, following Iran's addition of more tankers to its fleet and the easing of a shipping shortage that had delayed deliveries to its buyers. But analysts also said one month's number wasn't indicative of any trend.

"I think one month's comparison is not enough to figure out what happened," a dealer at trading house in Singapore said, declining to be identified because he is not authorised to talk to the media. "If you see the numbers, November was down but imports rebounded in December. It's all based on the lifting plans."

China—along with other main buyers of Iranian crude, including India, Japan and South Korea—has been under pressure from the US and Europe to steadily reduce Iranian imports. The West says Iran's enrichment of uranium demonstrates its intent to develop a nuclear weapons capability, an allegation Tehran denies.

China is Iran's top trading partner and has repeatedly voiced its opposition to unilateral sanctions outside those sanctioned by the UN.

But shipping delays—along with the contract disagreement in the first quarter—reduced China's crude imports from Iran last year by 21% from a year ago to 438,448 bpd.

China may reduce its purchases by a further 5 to 10% in 2013, according to preliminary indications, industry sources said. That would amount to a reduction of 20,000 bpd to 40,000 bpd, according to Reuters calculations.

China's Iranian crude import figures could also be pulled down by a price increase for South Pars condensate that was agreed to last year. Iran changed the pricing of its 2013 term exports of the condensate to China's top refiner, Sinopec Corp, effectively raising the premium on sales of the super light crude to its top client, Chinese industry sources said in January.

The price increase had led some Sinopec refineries to moderately reduce contracted volumes for 2013, although it is not known by how much. Sinopec imported about 70,000 bpd of the South Pars condensate last year.

Other top importers of Iranian crude on Friday also showed sharp drops from January last year.

South Korea posted a 16.1% fall from a year earlier to 190,200 bpd, data from state-run Korea National Oil Corp showed, while India imported 286,400 bpd of Iranian oil in January, down 41.2% from a year ago, according to data from trade sources.