FRANKFURT/VIENNA (Reuters) - Raiffeisen Bank International was warned by the U.S. Treasury in writing that its access to the U.S. financial system could be curbed because of its Russia dealings, according to a person who has seen the correspondence.

On May 6, Deputy Secretary of the Treasury Wally Adeyemo sent a letter to RBI, expressing concern about RBI's expansion in Russia as well a $1.5 billion deal with a sanctioned Russian tycoon that the bank has since aborted, according to the person, who requested anonymity because the matter is private.

While the deal linked to Oleg Deripaska was ditched by Raiffeisen days after the letter arrived, the source said that U.S. Treasury's concerns over Raiffeisen's business in Russia remain.

The warning is the strongest yet to the biggest Western bank in Russia and follows months of pressure from Washington, which has been looking into the Austrian lender's business in the nation for more than a year.

A spokesperson for Raiffeisen said that it had walked away from the Strabag deal and had not entered into any such transactions.

The spokesperson said RBI had "significantly reduced" activities in Russia and taken broad measures to mitigate the risks from sanctions.

"RBI will continue to work towards the deconsolidation of its Russian subsidiary," the spokesperson told Reuters.

In the letter, Adeyemo, the U.S. Treasury's second-highest ranking official, said Raiffeisen's expansion would contradict assurances RBI had given to the Treasury that they were trying to wind down in Russia, according to the source.

Adeyemo warned that RBI's actions increased the risk of Treasury needing to take action to restrict its access to the U.S. financial system given concerns that its behaviour put U.S. national security at risk.