Ukraine claims that it now has proof that Russian soldiers have been involved in fighting on Ukrainian soil:
Ukraine released video footage on Tuesday of what it said were 10 captured Russian soldiers, raising tensions as President Vladimir V. Putin of Russia arrived in Minsk, the capital of Belarus, for talks later in the day with his Ukrainian counterpart, President Petro O. Poroshenko.
....The release of the videos and the high-level talks came a day after Ukraine accused Russia of sending an armored column across the border, prompting Geoffrey R. Pyatt, the United States ambassador to Ukraine, to express alarm on Twitter. “The new columns of Russian tanks and armor crossing into Ukraine indicates a Russian-directed counteroffensive may be underway. #escalation,” he wrote.
....“Everything was a lie. There were no drills here,” one of the captured Russians, who identified himself as Sergey A. Smirnov, told a Ukrainian interrogator. He said he and other Russians from an airborne unit in Kostroma, in central Russia, had been sent on what was described initially as a military training exercise but later turned into a mission into Ukraine. After having their cellphones and identity documents taken away, they were sent into Ukraine on vehicles stripped of all markings, Mr. Smirnov said.
This kind of thing represents a cusp of some kind. If it's true, Putin has to decide pretty quickly whether to gamble everything on an outright invasion, or whether to back off. If it turns out to be a Ukrainian invention, Putin has to decide whether to use it as a casus belli. These are dangerous times.
UPDATE: Apparently Russia has admitted the soldiers are theirs:
Sources in Moscow have admitted that a number of men captured inside Ukraine were indeed serving Russian soldiers, but said they crossed the border by mistake...."The soldiers really did participate in a patrol of a section of the Russian-Ukrainian border, crossed it by accident on an unmarked section, and as far as we understand showed no resistance to the armed forces of Ukraine when they were detained," a source in Russia's defence ministry told the RIA Novosti agency.
Uh huh. I suppose Putin will now claim that detaining the soldiers is an act of war unless they're immediately released.