President Obama outlined a new package of sanctions against Russian firms and individuals on Wednesday.
"These sanctions are significant but also targeted," Obama said. "Russia will see that its actions in Ukraine have consequences."
The administration targeted large banks, as well as energy and defense firms. The sanctions stopped short of covering entire sectors of the Russian economy.
Obama said they were designed to inflict pain on Russia without harming U.S. companies or the nation's allies.
He said Russia had failed to heed calls that it stop weapons from flowing into Ukraine.
"So far, Russia has failed to take any of the steps that I have mentioned," Obama said. "In fact, Russia's support for the separatists and violations of Ukraine's sovereignty has continued."
The new sanctions bar American individuals and companies from trading in equities with those firms or extending them credit for periods longer than 90 days.
Among the firms targeted were Rosneft, the largest Russian oil producer; Novatek, a natural gas producer; Gazprombank, the financial arm of natural gas company Gazprom (though not Gazprom itself); and Vnesheconombank, or VEB, an economic development lender.
The administration also targeted eight state-owned defense firms and four Russian government officials, including an aide to President Vladimir Putin, the head of its Federal Security Service and the minister for Crimean affairs, as well as rump groups in breakaway areas within Ukraine.
The new sanctions go beyond earlier ones, which imposed travel restrictions on individuals and froze their assets.
"These are serious sanctions. They target major Russian energy companies and financial institutions," Steven Pifer, the former U.S. ambassador to Ukraine, told The Associated Press.
The Russian firms singled out by the administration will now be blocked from U.S. capital markets for financing medium and long-term debt. They will also have to look elsewhere to find dollars, which are an important financing tool.
"This is a significant step," a senior administration official said in a conference call with reporters. "Today's steps will only further exacerbate Russia's economic problems."
Administration officials emphasized that the Treasury Department, using authority granted by a series of executive orders signed by the president, could extend further sanctions.
European Union leaders, meeting Wednesday in Brussels, imposed fresh sanctions on Russia that did not match the scope of the latest moves by the Obama administration.
"There has been a little bit of a yin-yang where sometimes we're catching up with them and sometimes they're catching up with us," said another senior administration official.