A third VLCC of North Sea Forties crude oil is set to be arbitraged to South Korea in the fourth quarter of this year, trading sources said Thursday.

Unlike the previous two spot purchases, South Korea's SK Energy bought the third cargo in a swap deal, traders said.

A Middle Eastern sour crude trader said the arbitrage moves of North Sea crude into Asia could pressure the Middle Eastern sour crude market in the coming months.

South Korea is the world's fifth-largest importer of crude oil.

SK Energy declined to comment on the reported purchase Thursday.

The swap deal involved the purchase of around 2 million barrels of Forties on a delivered basis into South Korea in return for around 2 million barrels of Iraq's Basrah Light, including a price differential.

Traders said South Korea's other two main refiners, GS Caltex and Hyundai, have also bought cargoes of Forties for loading from Hound Point, Scotland, in the fourth quarter.

The sale to Hyundai was heard done by BP, and the sale to GS Caltex was heard done by Shell.

Traders at Shell and BP declined to comment on the reported trades.

There was a similar deal this month. Over the weekend, Unipec loaded a VLCC at Hound Point bound for South Korea, according to traders and ship tracking software.

The route between Hound Point and South Korea was a common arbitrage route for North Sea Forties crude between December 2011 and this March, when proposed South Korean tax reforms led to a drop in interest from refiners.

South Korea has a free trade agreement with the EU allowing refiners to import crude oil from EU countries such as the UK without paying import duty. The equivalent of 14 VLCCs of Forties crude were exported from the UK to South Korea in 2012.

European traders Wednesday said the arbitrage movements were supportive of backwardation in Brent spreads.

Forties is one of the four grades of physical North Sea crude oil that underpin Platts Dated Brent assessments, a key part of what ICE Brent crude futures settle against.