By David Enrich, Gabriele Steinhauser and Matthew Dalton

The economic sanctions that the U.S. and European Union imposed Tuesday against Russian banks were designed to severely punish a key part of Russia's economy. But their immediate impact is likely to be relatively muted.

The U.S. sanctions don't include Russia's biggest bank, and the EU carved out a loophole for big parts of the banks.

The sanctions on a variety of Russian companies in the banking, energy and arms industries were intended to penalize Russia for what the U.S. and EU see as its destabilizing role in the Ukraine conflict.

The U.S. slapped sanctions on three state-controlled Russian banks: VTB Bank, Bank of Moscow and Russian Agricultural Bank. The sanctions, which bar U.S. institutions and individuals from providing new equity or debt financing to those lenders, "will impose additional significant costs on the Russian Government for its continued activities in Ukraine," the U.S. Treasury Department said Tuesday.

Analysts said Wednesday that by largely locking the banks out of western capital markets, the U.S. and EU sanctions will drive up the lenders' funding costs, hurting their profits and potentially impeding their ability to lend.

The U.S. sanctions, however, are notable for an institution that is not on the list: OAO Sberbank. Sberbank, majority-owned by the Russian government, is the country's dominant lender. With more than 19,000 branches, it controls a plurality of Russia's banking assets and boasts the highest market share in retail, business and corporate banking. It also has substantial investment- and corporate-banking activities throughout Europe.

Sberbank's Moscow-listed shares rose about 3% Wednesday.

While the EU hasn't yet announced its list of sanctioned banks and other institutions, Sberbank is among them, according to EU officials. The bloc's list of sanctioned institutions, which includes five state-owned banks, will be published later this week.

But the EU sanctions also have a notable gap. The EU subsidiaries of the Russian banks are exempted from the sanctions, according to European officials familiar with the sanctions.

It is an important loophole. VTB Bank and Sberbank both have sizable units in Austria, and VTB also has a Cypriot subsidiary. Combined, those units at the end of last year had well over EUR20 billion of assets, and they operated in Austria, Cyprus, Czech Republic, Slovakia, Hungary, Germany and France.

If the sanctions had applied to the entirety of the banks, it could have made it much harder for those EU units to finance themselves, although EU officials said Wednesday that much of their financing was coming from their Russian mother companies.

And it likely would have created complications from the European Central Bank, which this fall will take on responsibility for supervising the largest euro-zone banks, including VTB's and Sberbank's units headquartered in Austria and Cyprus.

The exemption of the banks' EU arms was a late addition to the planned sanctions, only appearing in draft language in recent days, according to the European officials.

EU officials said Wednesday that they excluded the continental subsidiaries to protect the stability of Europe's financial system. The officials said the EU subsidiaries will be allowed to transfer certain funds to their parent companies, but not money that they raised in the EU. The officials said the goal of the sanctions isn't to hurt the banks themselves but to undermine their ability to finance the Russian economy.

Another factor blunting the impact of the U.S. and EU sanctions is that the Russian banks don't have much debt maturing in coming months.

Russian banks have roughly $39 billion of debt held outside Russia maturing in the next nine months, equivalent to 8% of their cash reserves, according to Nomura.

That suggests refinancing risks are manageable. For instance, VTB has potentially $3 billion of debt that needs to be refinanced before the end of the year, according to Bank of America Merrill Lynch. Sberbank has about $2 billion.

"We think that such low exposure limits the potential of debt-market deterioration on the banking sector, at least in the short term," said Vladimir Osakovskiy, Russia economist at Bank of America.

"The decision taken by the U.S. to restrict VTB's access to the capital markets will not impact the bank's operations and its ability to meet its ongoing obligations," the bank said in a statement. VTB added that it "adheres strictly to all relevant international laws and regulations."

Sberbank had no immediate comment.

Although the sanctions only directly restrict the banks' abilities to find western buyers for new debt and equity, the actual implications could be broader. U.S. and European financial institutions generally are reluctant to do any business with sanctioned entities. That wariness has grown recently after the U.S. severely penalized a series of European banks for financing entities in sanctioned countries.

Ben Edwards in London, Laurence Norman and Matina Stevis in Brussels and

Gregory L. White

in Moscow contributed to this article.

Write to David Enrich at david.enrich@wsj.com, Gabriele Steinhauser at gabriele.steinhauser@wsj.com and Matthew Dalton at Matthew.Dalton@wsj.com