Ukrainian leaders fear a Russian attack on their country is near.
After Moscow officially annexed the Crimea last week, Russian troops took two Ukrainian military bases by force this weekend and held 80 Ukrainian Marines captive.
In addition, 20,000 Russian troops remain massed on Ukraine's eastern border.
Thousands of demonstrators in Kiev waved Ukrainian flags in a city square, hoping that the United States and Europe will go beyond words and even economic sanctions to repel the Russian invaders.
While Putin's popularity soars in Moscow polls, U.S. lawmakers took verbal shots at him on Sunday talk shows, knowing that short of war, Washington doesn't have a good hand to play
"He is a bully, and we've got to call him for what he is," Sen. Richard Durbin, D-Illinois, said on CBS "Face the Nation" Sunday. "But this notion that some sanction is going to stop a former colonel in the KGB from his ambitions of a Russian empire is naive."
A bipartisan team from the Senate Armed Services Committee made a show of solidarity with Ukrainian leaders this weekend. Sen. Kelly Ayotte, R-N.H., suggested restoring missile defense agreements in Europe cancelled during President Barack Obama's first term.
"I believe that we should revisit the decision President Obama made when he came into office with regard to the missile defense systems in Poland and the Czech Republic," Ayotte said.
But that wouldn't do much for the people of Ukraine, who have lost the Crimean region of their country and could soon see more military action in the pro-Russian east and south.
The crisis prompted a meeting between President Obama and other world leaders gathered for a nuclear summit in The Netherlands.
"We (the G7 nations) are united in imposing a cost on Russia for its actions so far," the president said in Amsterdam Monday.
Sen. Joe Donnelly, D-Ind., said he believes Putin has angered and awakened Europe to the dangers of a Greater Russia.
Donnelly cautioned in Kiev that Putin "has stirred up countries all throughout the region who have said, 'We will never be dependent on President Putin for natural gas or energy needs or anything else, ever again.'"
But indignation from the West was not enough to prevent officials in Moscow from installing the flag of Crimea in the Russian parliament Monday.
There may be more flags to come if Putin sets his sights on more Ukrainian territory.