(CNN) -- Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov have practically been joined at the hip in the past week or so.
The two met four times last week in Europe and have been in daily phone contact since. But they have failed to reach common ground on how to solve the crisis in Ukraine over Crimea.
Kerry left Thursday for yet another round of talks with Lavrov -- this time, an 11th-hour push in London aimed at averting an escalation of the situation over a referendum for the strategic peninsula to rejoin Russia.
The stakes are higher as the clock ticks down to Sunday's referendum.
Before leaving Washington, Kerry told a Senate Committee that Russia would face an immediate "very serious series" of steps from the United States and Europe if the referendum goes forward and Russia takes moves to annex the disputed region.
He said he had spoken to Lavrov before the hearing and hoped that "reason would prevail." But he stressed there were no guarantees.
"My hope is they will become aware of the fact that the international community is really strongly united," Kerry said.
He suggested that during his talks with Lavrov, he would press Russia to agree to "something short of a full annexation" of Crimea but offered no details.
He suggested a diplomatic effort could continue even if the referendum was approved and if the Russians did not annex Crimea.
A senior State Department official followed up by saying Kerry would ask Lavrov to use his influence to buy time and space for negotiations to take place.
"We are going to give diplomacy every chance."
The official said the United States would present the best offer for deescalation that the Ukrainian people can accept and see if the Russians will take that course.
Tightening their grip
But Russia is showing no sign of backing down.
Pro-Russian forces are tightening their grip in Crimea. Well armed men have effectively isolated the Crimean peninsula, which has an ethnic Russian majority, from the rest of Ukraine.
And Moscow's defense minister announced Thursday the start of massive artillery drills near Russia's southern border with Ukraine involving 8,500 troops and a large amount of hardware.
The question now: Is the increased show of military might ahead of Sunday's vote in preparation for an annexation of Crimea or a daring move in a game of chicken ahead of Sunday's vote?
The potential price for Russia is steep.
The Senate is weighing legislation that could impose economic penalties on Russians involved in its intervention in Crimea. The measure would represent some of the toughest sanctions on Moscow since the end of the Cold War.
Although there are currently no sanctions against Russian banks or energy companies, lawmakers and U.S. officials have suggested that could be in the offing over time if Russian aggression continues.
Last week, President Barack Obama issued an executive order slapping visa bans on Russian and Ukrainian officials involved in the intervention and laying the groundwork for financial sanctions against those responsible for the crisis.
The European Union has been divided over how quick and severe the sanctions should be, but European officials say they are considering travel bans, asset freezes, and possibly sanctions against Russian companies and banks if Russian aggression continues.
'It can get ugly fast'
Kerry employed unusually tough language Wednesday in testimony before a House panel when he spoke about the measures the United States and its European allies were ready to take against Russia without a "reasonable outcome."
"It can get ugly fast if the wrong choices are made, and it can get ugly in multiple directions," he said.
But along with the prospect of being hit with fairly large stick, the United States and its European allies along with Ukraine's acting prime minister is offering possible carrot -- a political solution that could lead to more autonomy for Crimea if Russian troops withdraw.
Alongside Obama in the Oval Office, Ukraine's pro-Western interim Prime Minister, Arseniy Yatsenyuk, struck a bold and defiant tone, saying his country would "never surrender" it's sovereignty.
Obama vowed to "stand with Ukraine."
But there are signs of a potential diplomatic opening. Obama implied there was a formula for giving Russia a greater voice in Crimea.
"There is a constitutional process in place and a set of elections that they can move forward on that, in fact, could lead to different arrangements over time with the Crimean region," Obama said. "But that is not something that can be done with the barrel of a gun pointed at you."
A willingness to compromise?
During an appearance at the Atlantic Council on Wednesday, Yatsenyuk was more forward leaning on his willingness to compromise with Moscow over Crimea.
"We the Ukrainian government are ready to start a nationwide dialogue how to increase the rights of autonomous Republic of Crimea, starting with taxes and ending with other aspects like language issues," he said.