The world's second-largest economy is the top importer of crude oil, with its refining operations mainly serving the country's massive domestic market.

It also exports various refined oil products including gasoline, diesel and jet fuel that totalled 58 million tonnes last year, according to official customs statistics.

However, because of the war, "Chinese oil refiners have begun cancelling agreed refined fuel export cargoes", Bloomberg News said, citing unidentified people familiar with the matter.

The latest instructions are "a step up" from last week's guidance to suspend shipments, which "had been understood as non-mandatory", according to the story.

Asked about the report at a regular news conference on Thursday, foreign ministry spokesman Guo Jiakun said he was not familiar with the situation.

Global energy markets have been shocked since the start of the US-Israel war on Iran, with oil soaring above $100 a barrel on Thursday as Tehran's attacks on Gulf states overshadowed a record release of strategic crude by the International Energy Agency.

China is not a full member of the IEA and is therefore not obligated to participate in coordinated release.

The IEA's announcement did not allay concerns over the choking of energy supplies from the Middle East. The Strait of Hormuz, through which roughly a fifth of global crude passes, is effectively closed.

More than half of China's total seaborne crude imports last year came from the Middle East, according to analytics firm Kpler.

However, analysts say years of strategic stockpiling will allow it to withstand disruptions, at least in the near term.

China holds about 1.2 billion barrels in onshore crude inventories, equivalent to about 115 days of what it imports by sea, Kpler says.

Beijing authorised a release from its massive oil reserves in 2021 in response to spiking factory gate inflation.

The body that announced that decision -- the National Food and Strategic Reserves Administration -- has so far not issued similar measures in response to the current market turmoil.

Foreign ministry spokesman Guo said this week that "China will do what is necessary to protect its energy security".