Oil prices have continued their rally, jumping to near six-month highs after the US cracked down on Iranian oil exports.

The White House announced on Monday that it was calling an end to six-month waivers that had exempted countries from unilateral US sanctions on Iranian oil exports.

Starting in May, these countries – China, India, Turkey, Japan, South Korea, Taiwan, Italy and Greece – would face sanctions if they continue to buy the oil.

"This points to a big drop in the supply side, which boosts the commodity's price. Iran's daily oil output amounts to 1.3 million barrels, according to latest figures in end March," said Margaret Yang Yan, market analyst at CMC Markets Singapore.

But she said that "the sustainability of oil's rally depends on Saudi and other OPEC members' actions to increase oil supply in the month to come."

Stephen Innes, head of trading and market strategy at SPI Asset Management, said rising prices meant $80 a barrel was now a "possibility".

"Oil quickly repriced higher on fears that markets could face an immediate supply crunch, adding more pressure to the already tenuous global supply squeeze," he added.

The price of a barrel of Brent crude – the international benchmark – stood at over $74 in early trade.

Energy and oil-linked shares also climbed.