U.S. officials say Russian fighter jets have flown into Ukrainian airspace several times in the last day or so.
It's not clear what the intent was. The aircraft could have been testing Ukrainian radar or making a show of force.
Meanwhile, tens of thousands of Russian troops continue to amass along the Ukrainian border as Russia increases its military exercises.
What's the likelihood of a direct armed confrontation between Russia and Ukraine? CBN News International Correspondent George Thomas explains more, on CBN Newswatch, April 25.
Pro-Russia forces also continue to occupy government buildings in Eastern Ukraine, despite an agreement that Russia signed last week in Geneva calling for Moscow to remove the forces.
Ukraine Interim Prime Minister Arseniy Yatsenyuk said Friday that Russia is keen to start World War III.
"Russia's goals are as follows: first, to thwart the presidential election in Ukraine so that it doesn't have a legitimately elected president," he said. "Second, to oust the pro-European and pro-Ukrainian government; and third, to seize Ukraine both militarily and politically."
The crisis in Ukraine has followed President Barack Obama on his trip to Asia. At a press conference in South Korea, he discussed the stakes for Russian President Vladmir Putin.
"There's going to come a point when he's going to have to make a fundamental decision," Obama said. "Is he willing to see an economy already weakened further or is he going to use military force and the kind of destabilizing activities that we see so far?"
Sen. Carl Levin, D-Mich., chairman of the Senate Armed Services Committee, visited Ukraine this week and warned of further financial penalties for Russia.
German Chancellor Angela Merkel said Friday that the European Union's foreign ministers will meet soon to consider further targeted sanctions against Russia.
President Obama said the United States and Europe are also considering sanctions for broad sectors of Russia's economy if Moscow invades eastern Ukraine.