LONDON, Jan 26 (Reuters) - Iranian oil exports are climbing in January after a boost in the fourth quarter despite U.S. sanctions, three assessments showed, in a sign that the end of Donald Trump's term as U.S. President may be changing buyer behaviour.

Iran's exports have shrunk since Trump in 2018 withdrew the United States from a nuclear deal with Iran and reimposed sanctions. New President Joe Biden has said if Tehran resumed compliance with the deal, Washington would too.

Figures from SVB International and two other firms that estimate Iranian supply by tracking tankers indicated exports are rising.

SVB International estimated Iranian crude exports increased to 710,000 barrels per day (bpd) in December from 490,000 bpd in October, and that shipments and production so far in January were moving up.

"Iran and customers are expecting an easier approach from the Biden administration," said Sara Vakshouri of SVB. "Iran has already started increasing its production and exports in anticipation of negotiations with the U.S."

Tehran and some of its crude buyers such as China have said U.S. sanctions are illegal.

Geneva-based Petro-Logistics said Iranian crude exports were set to exceed 600,000 bpd this month for the first time since April 2019, after rising by 100,000 bpd in the fourth quarter.

"We've seen a steady increase in Iranian exports over the past months," Petro-Logistics said in an email to Reuters.

While still a fraction of the pre-sanctions rate of about 2.7 million bpd in May 2018, that level of January exports represents a sizeable boost.

Refinitiv Eikon tracked just 370,000 bpd of Iranian crude exports in December. In some months of 2020, exports fell to as little as 100,000 to 200,000 bpd.

HARD TO TRACK

Iran's oil minister, Bijan Zanganeh, last week said crude exports had recently risen "significantly" without giving figures. Elias Naderan, an Iranian lawmaker, was quoted by Mehr news agency as saying they had reached 900,000 bpd.

There is no definitive figure for Iranian exports. Tankers have satellite tracking but this can be switched off, and adjustments to schedules and week-by-week variations complicate the task.

Iran has said documents are forged to hide the origin of its cargoes, and it is sometimes hard to know the final destinations of vessels due to oil being transferred ship-to-ship.

A deal that officially allows higher exports does not look imminent. Biden's choice for U.S. secretary of state, Antony Blinken, says the United States is some way from a decision on resuming the Iran nuclear accord.

A source at a third company that tracks Iranian exports put January shipments at no more than 500,000 bpd, still an increase from December, and said the behaviour of key buyers could change in coming months.

"Now Trump is gone the question arises whether China and India will continue to restrain purchases," he said.