KIEV, Ukraine — Ukraine and Russia traded increasingly bitter accusations of cross-border hostilities on Tuesday, deepening a shadowy war of real attacks or orchestrated sabotage that increasingly threatens to draw the two countries into direct conflict.

On Tuesday, Ukrainian military officials said they suspected Russia of carrying out an airstrike that destroyed a four-story apartment building in the eastern Ukrainian town of Snizhne, about 12 miles from border, killing at least 11 civilians. Pro-Russian separatists, in turn, said the Ukrainian military had carried out the bombing.

The announcement by Ukraine’s general prosecutor’s office that it was collecting evidence of a Russian role in the airstrike came a day after the government in Kiev said it believed Russia was responsible for the downing of a military transport plane in Luhansk. A day before that, the Russian Foreign Ministry warned of potentially “irreversible consequences” after one man was killed and two other people were wounded when mortar fire hit the town of Donetsk on the Russian side of the border.

Officially, the Kremlin has denied arming, financing or directing the insurrection in eastern Ukraine, but its active support of the rebellion has been openly acknowledged in recent days. Separatist leaders have complained about the low quality and advanced age of the weapons provided by Russia and a lack of more proactive assistance as they have come under heavier attack by the Ukrainian military.