An Iranian supertanker with two million barrels of oil is heading to Asia after sitting in Iranian waters for months, the first vessel storing crude offshore to sail after a nuclear deal this week, data showed on Thursday.

Iran and six major world powers reached a landmark nuclear deal on Tuesday, clearing the way for an easing of international sanctions on Tehran and higher oil exports.

While oil analysts do not expect Iran to make a major return to the market until next year, it has been parking millions of barrels of oil on tankers for months.

The fully laden Starla, operated by Iran's top tanker group NITC, had been used for floating storage since December 12, a tanker tracking source said.

"This is the first tanker to come off floating storage," the source said. "One of the scenarios is it could do an STS operation, although nothing is known at the moment," the source said, referring to ship-to-ship transfers of oil between two vessels, usually at sea.

ThomsonReuters Eikon shipping data showed the vessel sailing through the Gulf of Oman with a Singapore destination. Earlier this week the Starla was idling offshore the United Arab Emirates in an area known as Khor Fakkan, often used for STS.

It is unclear whether the estimated 2 million-barrel cargo had been sold, or if so whether the deal occurred after this week's agreement. But it is a milestone following a months-long build-up of idling crude tankers holding up to 50 million barrels of oil, equal to over a month's worth of Iran's exports.

Iran is "likely assuming that either a small increase in exports will not undermine the historic accord reached, or, that no one would notice," analysts at Macquarie wrote in a research note. However, if an increase in exports is sustained it will "weigh on near term balances."

Iran's Oil Minister Bijan Zanganeh said last month the country was aiming to add 500,000 barrels per day (bpd) to production within two months of Western sanctions being eased, and as much as one million bpd in six to seven months.

The sanctions have halved Iran's shipments to as little as one million bpd.

Years of under investment mean Iran may struggle to get its oil industry anywhere near full potential, analysts say. It will also take time to raise output while nuclear inspectors verify Iran's compliance with the terms of the deal, and sanctions are slowly removed.

Last month, tanker tracking sources said Iran was storing as much as 40 million barrels of oil, mostly crude, on board tankers at its anchorages, which could flood the oil market.

Windward, a Tel Aviv operated maritime data and analytics company, estimated this week that Iran was storing 51.4 million barrels of crude and condensate on 28 vessels at sea.

ThomsonReuters Research and Forecasts freight analysts put the figure slightly lower at up to 41 million barrels, according to a report last week.

Condensate is a type of very light crude oil and can be used as a diluent for extra heavy crude and as a feedstock for petrochemical plants and refineries. - Reuters