Russia's central bank is promising to support financial institutions hit by US sanctions as stocks took a tumble on opening in Moscow.

In an online statement, the bank promised to "take adequate measures" to support targeted institutions. Russia's state-owned VTB bank - Russia's second-largest - was trading down 1.2 percent on Wednesday morning.

Other major banks that were left unscathed by sanctions - such as the country's largest, Sperbank - were trading higher.

US officials said on Tuesday that roughly 30 percent of Russia's banking sector assets are now constrained by sanctions.

AP