The United States is sending Secretary of State John Kerry to London for a meeting with Russian Foreign Minister Sergei Lavrov, part of the effort to end the crisis in Ukraine.
America's top diplomat said the face-to-face meeting will take place Friday and is just the latest in a series of conversations he and President Barack Obama have had with Russia officials.
"While we respect obviously that Russia has deep historical, cultural and other kinds of interest with respect to Ukraine and particularly Crimea, nothing justifies a military intervention that the world has witnessed," Kerry said.
Kerry said the U.S. and its European partners have made clear to Moscow there are many ways to address its concerns without further escalation. And he said the U.S. does not believe it would serve anyone's interests to isolate Russia.
Still, he issued a warning.
"We will do what we have to do if Russia cannot find a way to make the right choices here," he said.
Kerry's comments come on the day President Barack Obama meets with Ukraine's interim prime minister, Arseniy Yatsenyuk, at the White House. The secretary's trip comes ahead of a planned referendum Sunday in Crimea on whether to join Russia.
Analysts say the meeting will not stop the referendum, but warn of the urgency of stopping Russia from moving on to other Russian-populated regions of the former Soviet Union.
Russia has made it clear that a flurry of diplomatic activity ahead of the Sunday referendum will have little effect on its plans to take control of the Crimean region. Washington-based political analyst Peter Eltsov said that no matter what official name it will assume, Crimea is lost to Ukraine. He added Ukraine has to fight to prevent any Russian attempt to move further.
"It's the biggest fear of the new Ukrainian government and it is quite likely - depending of course on the political situation - that [Russian President Vladimir] Putin will try to go to certain parts of eastern Ukraine. We need to remember that there is no direct by-land connection between Russia and the Crimean peninsula," Eltsov said.
Stephen Blank, an analyst with the American Foreign Policy Council, agreed. He also placed blame on the European Union for a lax response to Russia's move to take over the strategic peninsula.
"There have been no real organized economic sanctions on Russia; there have been no systematic strategic military actions to strengthen Ukraine’s ability to defend itself; and if I were Mr. Putin I would think I’ve gotten away with it. I don’t think he will in the end, but I think up till now there has been too little action, and whatever action there has been, has been uncoordinated," Blank said.
VOA’s Zlatica Hoke contributed to this report.