(Adds Power quotes, McCain, latest sanctions, byline)
By Doina Chiacu
WASHINGTON, April 13 (Reuters) - The United States is prepared to step up sanctions against Moscow if pro-Russian military actions in eastern Ukraine continue, the U.S. envoy to the United Nations said on Sunday.
Pro-Russian activists seized government buildings on Saturday in the eastern town of Slaviansk, about 150 km (90 miles) from the Russian border. Ukrainian security forces were trying to oust the activists, who set up barricades on the outskirts of the city.
The American ambassador to the U.N., Samantha Power, said on ABC's "This Week" that the latest events in Ukraine bore "the telltale signs of Moscow's involvement."
She said sanctions already imposed by Washington have had an impact: the Russian rouble has fallen to an all-time low, the country's stock market has depreciated by 20 percent and investors are fleeing the country.
"The president has made clear that, depending on Russian behavior, sectoral sanctions in energy, banking, mining could be on the table, and there's a lot in between," Power said.
"I think we've seen that the sanctions can bite, and if actions like the kind we've seen over the last few days continue, you're going to see a ramping up of those sanctions."
The sanctions have been the most visible sign of U.S. anger at Russia's annexation of the Crimea region in southern Ukraine last month, reflecting the deepest plunge in U.S.-Russian relations since the Cold War.
Ukraine now faces a rash of rebellions in the east that it says are inspired and directed by the Kremlin.
Asked on ABC if Russian President Vladimir Putin wants to seize eastern Ukraine, Power said his actions "give credence to the idea."
Though Russians are insisting that is not what Moscow wants, she said, "Everything they're doing suggests the opposite."
NATO described the appearance in eastern Ukraine of men with specialized Russian weapons and identical uniforms without insignia -- as previously worn by Moscow's troops when they seized Crimea -- as a "grave development."
Power said the rebellion has "all the telltale signs of what we saw in Crimea: It's professional, it's coordinated, there's nothing grassroots-seeming about it. The forces are doing in each of the six or seven cities that they've been active in exactly the same thing."
Republican Senator John McCain, a frequent critic of U.S. policy on Ukraine, said on CBS that the Obama administration's failure to punish Russia over Crimea has only emboldened Putin.
"The question is now, What do we do and what does he do?" McCain said on "Face the Nation." "It's obvious that he is encouraged by the fact that we sanctioned a few people and suspended -- didn't even throw him out -- of the G8."
McCain repeated his calls for tougher sanctions and for giving Ukrainians light weapons so they can defend themselves.
"They didn't fight in Crimea," he said. "But if he starts moving in further encroachment in this way into eastern Ukraine, they will fight."
The United States on Friday imposed sanctions on a Crimea-based gas company, Chernomorneftegaz, effectively putting it off limits to Russia's state-controlled Gazprom, which was expected to bid for a stake in the company.
The move, along with penalties on six Crimean separatists and a former Ukrainian official, is the third round of U.S. sanctions since the Ukraine crisis erupted. (Reporting by Doina Chiacu; Additional reporting by Sarah N. Lynch; Writing by Doina Chiacu; Editing by Jim Loney and Leslie Adler)