Kerry said he has consulted other world leaders and all are committed to doing what is necessary to isolate Russia diplomatically. President Barack Obama spoke Sunday with German Chancellor Angela Merkel, British Prime Minister David Cameron and Polish President Bronislaw Komorowski.
Kerry planned to travel to Kiev on Tuesday for meetings with the Ukrainian government. Treasury Secretary Jacob Lew said the United States is ready to work with other countries and the International Monetary Fund to provide support for Ukraine's economy.
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.
Former Ambassador Nicholas Burns said, "Putin's not going to back off. ... What can President Obama do? Be very minded in opposition. We can't follow a military policy. This has to be diplomatic."
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.