* West sanctioned Russian diamond imports over Ukraine war

* Antwerp diamond dealers criticise implementation of ban

* Say they fear loss of competitive advantage

* Belgian city has been diamond trade hub for centuries

* Belgian official says disruptions are now easing

LONDON/BRUSSELS, March 14 (Reuters) - Antwerp's diamond dealers are facing long and expensive delays in their imports of the precious stones due to an EU ban on Russian-origin diamonds that came into effect on March 1, a letter seen by Reuters showed.

The letter, dated March 13, said the disruptions would further erode the competitive advantage of the centuries-old Antwerp diamond trade. It was addressed to Belgium's main diamond industry group, Antwerp World Diamond Centre (AWDC), and requested a review of the new procedures.

"While we fully support the decisions taken by Belgium, the European Union, and the G7 nations, in regards to the sanctions of January 1st 2024, the implementation of the measures to enforce the sanction has adversely affected all of our operations," said the letter, signed by over 100 local firms.

"The intention was to prevent the flow of diamonds from sanctioned states, but the reality we face is the severe disruption of our supply chains, and alienation from the rest of the global trade."

A Belgian government official said the delays were a temporary blip that were now easing.

The EU and Group of Seven (G7) countries agreed to ban direct imports of Russian diamonds to their markets as of Jan. 1 and before phasing in a full ban on Russian-origin stones via third countries from March 1 because of Moscow's war in Ukraine.

Russia's state-run Alrosa, among the world's top diamond producers, was also sanctioned by the EU.

DIAMOND HUB

Antwerp remains the world's biggest diamond hub though 90% of stones are now polished in India. Belgium pushed hard for the G7 to adopt a version of its proposed plan in an effort to prevent Antwerp from losing even more business after major Western jewellers began eschewing Russian stones.

Diamond dealers said their shipments have been held up for over a week at customs even if the gems were coming straight from African producers.

The Belgian government official said pending shipments would now be processed within 24 hours.

"The indirect ban coincided with the Hong Kong Diamond Fair which is an annual peak period... This, in combination with the expected teething problems caused some initial delay in processing of shipments during the first days," he said.

Diamond dealers say they expect more problems when the additional tracing requirements take effect from September.

"We see the procedures will cause Antwerp to further lose competitive advantage... rather than deal a meaningful blow to any sanctioned products," the letter said.

"The current trajectory threatens the existence of Antwerp's diamond industry, a heritage of six centuries."

The head of the AWDC, Ari Epstein, said the group would soon present the new measures, adding it was "acutely aware of the challenges and disruptions this timing may have caused".

"Let me be unequivocally clear: the violation of sanctions is criminal in nature and not taken lightly by governments or our organization. Our commitment to compliance... is unwavering and absolute," Epstein said in a statement. (Reporting by Dmitry Zhdannikov and Julia Payne Editing by Gareth Jones)