China has offloaded nearly four million barrels of Iranian crude oil into state reserve tanks in the southern port city of Zhanjiang over the past few weeks, a trade source and ship tracking specialist Vortexa Analytics told Reuters on Thursday.

China brought in 260,312 tonnes (1.9 million barrels) of Iranian crude oil in December, according to data from the General Administration of Chinese Customs, which last recorded Iranian oil inflows in December 2020 at double the volume.

The identities of the companies that brought the latest shipment and the port at which the cargo was offloaded were not revealed.

On Thursday, China announced its first imports of Iranian crude oil in a year, despite the unilateral sanctions imposed by the US government.

A few days ago, the Secretary of Iran's Supreme National Security Council Ali Shamkhani revealed that the implementation of a comprehensive strategic cooperation agreement between Iran and China for the next 25 years is a strategic success for Tehran.

The comprehensive strategic cooperation agreement between Iran and China was signed on March 27, 2021, in Tehran.

Asked about China storing Iranian oil in state reserves and noting it in customs data - effectively confirming violations of US sanctions - the US State Department said US sanctions remain in effect and would be enforced, though Washington believed diplomacy was the best way to address the issue.

"We are aware of the purchases that Chinese companies are making of Iranian oil. We have used our sanctions authorities to respond to Iranian sanctions evasion, including those doing business with China, and will continue to do so if necessary," said a State Department spokesperson who asked to remain anonymous.

"However, we have been approaching this diplomatically with the Chinese as part of our dialogue on Iran policy and think that, in general, this is a more effective path forward to address our concerns," the spokesperson added.

To avoid the sanctions, Iranian crude has been exported to China marked as oil from Oman, the United Arab Emirates, and Malaysia, squeezing out supplies from Brazil and West Africa, traders have said.

Imports from Iran have accounted for about 6% of China's crude oil imports, according to shipping data and traders' estimates.