Washington DC - The US on Thursday amended its sanctions relief offered to Russian oil tankers already at sea, saying shipments bound for Cuba and North Korea were still subject to restrictions.

In an amended general license, Washington continued to allow the sale of oil on Russian tankers at sea as of March 12, but said such transactions with its adversaries Cuba and North Korea were prohibited.

The original order had only made an exception for transactions involving Iran. Thursday's order broadened the scope of the exceptions, also including some Russian-occupied regions of Ukraine.

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On March 12, the US temporarily allowed the sale of Russian oil that is at sea, as energy prices soared after US-Israeli strikes on Iran plunged the Middle East into war.

The move marked a momentary easing of economic sanctions against Russia, which has been targeted over its invasion of Ukraine.

The Treasury license authorizes the delivery and sale of Russian crude oil and petroleum products that have been loaded on vessels on or before 12:01 AM ET on March 12, through 12:01 AM on April 11.

President Donald Trump has threatened Havana repeatedly since taking office, saying Monday that he would "take" the communist-run island.

Cuba has been under a US fuel blockade since shortly after Washington deposed Venezuelan president Nicolás Maduro, a key ally of Havana, on January 3.

The blockade has caused a dire economic crisis, and blackouts have been exacerbated by the sudden suspension of oil supplies.

No fuel has been imported to the island since January 9, hitting the power sector while also forcing airlines to curtail flights to the island, a blow to the all-important tourism sector.

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Thursday's move also came as a tanker carrying Russian diesel was set to arrive in Cuba "in several days" after using deceptive maneuvers to reach the island, according to a maritime tracker.

"If or when the tanker arrives, this will be the first confirmed arrival of a refined products cargo at the island since early January," maritime intelligence firm Windward reported online.

A second ship, the sanctioned Russian tanker Anatoly Kolodkin, is also on its way to Cuba carrying 730,000 barrels of crude oil, maritime analytics firm Kpler said on Wednesday.