In Brussels, NATO's secretary-general, Anders Fogh Rasmussen, said Russia's actions have violated a U.N. charter. He said the alliance was re-evaluating its relationship with Russia.
"There are very serious repercussions that can flow out of this," Kerry said.
Beyond economic sanctions and visa bans, freezing Russian assets, and trade and investment penalties, Kerry said Moscow risks being booted out of the powerful Group of Eight group of world powers as payback for the military incursion.
Several U.S. senators also called for bolstered missile defense systems based in Poland and the Czech Republic.
Russia is "going to be inviting major difficulties for the long term," said Kerry. "The people of Ukraine will not sit still for this. They know how to fight."
Still, it was clear that few in the West were prepared to respond immediately to Putin with military force.
At the Vatican, Pope Francis used his traditional Sunday midday appearance in St. Peter's Square to urge world leaders to promote dialogue as a way of resolving the crisis in Ukraine.
Sen. Marco Rubio, R-Fla., discussing the potential of U.S. military strikes against Russian troops in Crimea, said, "I don't think anyone is advocating for that." One of the administration officials indicated that the U.S. was not weighing military action to counter Russia's advances, saying the Obama administration's efforts were focused on political, economic and diplomatic options.
Rubio said it would be difficult to rein in Moscow. He said Putin has "made a cost-benefit analysis. He has weighed the costs of doing what he's done, and ... clearly he has concluded that the benefits far outweigh the costs. We need to endeavor to change that calculus."
As a starter, Rubio and fellow GOP Sen. Lindsey Graham of South Carolina said the Obama administration should return to plans it abandoned in 2009 to place long-range missile interceptors and radar in Poland and the Czech Republic.