President Barack Obama announced additional sanctions Thursday against high-level Russian officials and associates of President Vladimir Putin.
The president also warned of the possibility of even harsher penalties on what he calls "key sectors" of the Russian economy if Moscow doesn't take a step back in Ukraine.
The news comes as Russia is tightening its grip on Crimea, despite growing international pressure.
Ukraine's acting president said Russian forces have freed its navy commander, taken captive at navy headquarters.
What is Russia's role in biblical prophecy? Best-selling author Joel Rosenberg answers that question and more on Christian World News, March 21. Click play to watch.
Meanwhile, German Chancellor Angela Merkel told her parliament the G-8 summit will not meet in Sochi until the situation changes. She also said the EU will impose more sanctions on Russia and expand a freeze of bank accounts of Russian officials.
In the United States, the Pentagon announced it will participate in a planned multinational military exercise in Ukraine this summer.
"We are not going to be getting into a military excursion in Ukraine," Obama said.
"What we are going to do is mobilize all of our diplomatic resources to make sure that we've got a strong international coalition that sends a clear message, which is the Ukraine should decide their own destiny," he added.
Vice President Joe Biden traveled to the region this week to reassure Russia's nervous neighbors America will protect them if Moscow proceeds further.
Mob Attacks US Christians
Meanwhile, one group of U.S. Christians found themselves drawn into the rising tensions in eastern Ukraine when they came face-to-face with an angry mob of pro-Russia demonstrators.
On March 16, Gennadiy Mokhnenko and six Americans traveling on a bus in the southeastern Ukrainian city of Mariupol, not too far from Russia's border.
They had just finished ministering at a women's prison when dozens of pro-Russian demonstrators confronted them on the street.
"I was really afraid for my team," Mokhnenko recalled. "They were very aggressive and crazy. I can't believe this."
Mohknenko, who runs a Christian orphan ministry, said the demonstrators, who were chanting "Russia! Russia," tried to pull his team off the bus.
Fortunately, they managed to get away unhurt, but not before windows were smashed and their tires shot.
The incident, which was filmed and uploaded to YouTube, made the local news.
Mohknenko said Russia is sending men across the border to cities like Mariupol to instigate chaos and create a reason for a broader invasion of Ukraine.
"I have 32 adopted children," he said. "I am afraid for my children. We have a real chance for war here."
'No Place for Warmongers'
Meanwhile, Ukraine's Prime Minister Arseniy Yatsenyuk vowed such instigators would be dealt with.
"Law enforcement agencies have gathered convincing evidence of the participation of Russian special services in organizing unrest in the east of our country," he said. "There are saboteurs who have been arrested."
"There is no place in Ukraine for these warmongers," he added.
Violent protests have broken out in several key regions in mainly Russian-speaking eastern Ukraine.
In some cases, pro-Russian demonstrators have taken over government buildings, replacing the European Union flag with Russian flags.
For example, in Donetsk, north of Mariupol, hundreds of demonstrators stormed a government security building this week.
The clashes between those for and against Russia are becoming more frequent and violent, and the stakes couldn't be higher.
Russia is adding more troops and military equipment at its border with Ukraine, poised to easily take cities like Kharkov and Donetsk, and advance as far as Kiev the capital within two or three hours