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"We will ease these restrictions as soon as security conditions permit," the document says.

The letter was signed by NBU Governor Valeria Gontareva, Ukrainian President Petro Poroshenko, Prime Minister Volodymyr Groysman, and Finance Minister Oleksandr Danyliuk.

The Ukrainian authorities explained that the sanctions were imposed due to a large-scale public outcry and a negative reaction to the said banks' activity in Ukraine.

Read alsoRussian Sberbank's Ukrainian subsidiary to be sold for about $130 mln – media"The recent events also caused a strong negative public reaction to Russian state-owned banks operating in Ukraine. In the interest of our national security, we imposed restrictions on the transactions of these banks with their parent banks, and we have notified the IMF's Executive Board of these measures under Decision No. 144-(52/51)," the document said.

At the same time, the authors of the letter stress that the banks are solvent and continue abiding by prudential regulations. They guaranteed the safety of the banks' structural divisions in Ukraine and pledged necessary support for their liquidity.

"We have made arrangements to ensure that these banks have quick access to the necessary liquidity support if needed, including emergency liquidity assistance from the NBU," the letter said.

Read alsoNBU enforces sanctions against Russian banks' subsidiariesAs UNIAN reported earlier, the National Bank of Ukraine had imposed sanctions against the subsidiaries of Russian state-owned banks in Ukraine since March 23 in keeping with a decision by the National Security and Defense Council (NSDC). The sanctions banned any financial operations of these banks in favor of persons associated with them, in particular, their parent structures.

On March 16, President Poroshenko signed a decree enacting the NSDC decision on 12-months sanctions vis-à-vis the subsidiaries of Russian state-owned banks, namely Sberbank, Prominvestbank, VTB Bank, BM Bank, and VS Bank.

The NBU said that the sanctions should not be an obstacle to additional capitalization of the banks by their parent structures and their sale to new investors, as well as settlements of bank customers.