More than £14million of UK assets have been frozen over possible money laundering in Ukraine.
The Serious Fraud Office acted yesterday after suspicious activity involving certain businesses following the Russian invasion of Crimea and subsequent political turmoil in Ukraine.
"For reasons of confidentiality we cannot say more at this time," the SFO added.
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The EU has imposed travel bans and asset freezes on another 15 Russian officials.
Preparations for the third tier of sanctions, which will have far-reaching economic and trade measures, are "well advanced", said Foreign Secretary William Hauge who is due in Ukraine next week.
He warned the "doors of diplomacy remain fully open" but warned Moscow it will pay a "heavier price" if it fails to act to quell the unrest.