BBC journalists filed a somewhat breathless report this morning of having witnessed rebels shelling a Ukrainian army position east of Mariupol, in the embattled town of Shyrokyne. The OSCE has been witnessing engagements like these for the past several weeks, and Ukrainian soldiers interviewed by the BBC appear to have slept through the shelling, saying it was a routine occurrence. Notably, however, the BBC correspondent embedded with the Ukrainian troops said an unexploded shell looked to be the type fired by heavy artillery, which was supposed to have been removed from the front lines under the Minsk ceasefire agreement.This report comes on the heels of the U.S. State Department accusing Russia yesterday of strengthening air defenses in rebel-held eastern Ukraine, and of stepping up a training program which included the use of Russian drones. Russia had massed the largest number of troops on Ukraine’s border since October, a State Department spokeswoman said, adding that Russian activity within Ukraine was at its highest level since August.Is the Minsk ceasefire in danger? Though Russian Foreign Minister Sergei Lavrov said he believed that the worst of the recent skirmishing in eastern Ukraine, which has left at least seven Ukrainian soldiers dead in the past few weeks, was over, Ukrainian intelligence and NATO officials have been voicing concerns that the ceasefire would break down completely before May. If it comes to pass, the fight could pull in neighboring countries into the fray. The Polish Foreign Minister Grzegorz Schetyna, chafing under yesterday’s threats by Russian generals that his country would be directly targeted with nukes, said today that if there was a resumption of violence in Ukraine, it would prompt his country to consider providing “stronger support” for Ukraine’s defense.Russia has repeatedly shown that it can raise and lower the temperature in Ukraine at will—and it appears to be pushing the mercury higher right now. How high it wants to take it is anyone’s guess, but the situation on the ground in the east is clearly explosive these days, and could easily spiral out of control.