"There will be a response of some kind of the referendum itself and, in addition, if there is no sign of any capacity to be able to move forward and resolve this issue, there will be a very serious series of steps on Monday in Europe and here," Kerry told the Senate panel.
It was not clear, however, whether Russia would heed the warnings, and Moscow has refused demands by the West to pull back troops from Crimea and respect Ukraine's territorial boundaries. Under a long-standing security agreement with Ukraine, Russia is allowed to deploy up to 25,000 troops to the Crimean Peninsula, and has a large navy there.
"There are limits on how much blunt force, in terms of sanctions and isolation, will move somebody who doesn't seem to have been particularly responsive to that throughout his career," said John Norris, a security expert at the liberal-leaning Center for American Progress think-tank in Washington. He was referring to Russian President Vladimir Putin.
However, Norris said, the people around Putin, including Russian business leaders and wealthy citizens who travel to the West, may have more influence on Moscow if they start feeling squeezed by sanctions. "That may not be decisive in Putin's thinking right now, but I think it does begin to move the thinking of those around him," Norris said.
Kerry said he planned to make clear how high the stakes are when he sees Lavrov in London, and suggested he would press Russia to accept "something short of a full annexation" of Crimea — but did not elaborate on what that might entail.
"My hope is that they will come aware of the fact that the international community is really strong and united on this issue," he said.
In another show of support for Ukraine's sovereignty, Vice President Joe Biden met with Ukrainian Prime Minister Arseniy Yatsenyuk on Thursday, a day after the new prime minister met with President Barack Obama. The White House said Biden told Yatsenyuk that the U.S. "stands firmly behind Ukraine and the Ukrainian people in ensuring Ukraine's sovereignty and territorial integrity."