Depending on whom you believe, Russian troops either invaded Ukraine in the last few days and began firing at Ukrainian forces on Friday, or are not in Ukrainian territory at all.
From the New York Times:
The Russian military has moved artillery units manned by Russian personnel inside Ukrainian territory in recent days and was using them to fire at Ukrainian forces, NATO officials said on Friday.
The West has long accused Russia of supporting the separatist forces in eastern Ukraine, but this is the first time it has said it had evidence that the Russian military was operating in Ukrainian territory.
The Russian move represents a significant escalation of the Kremlin’s involvement in the fighting there and comes as a convoy of Russian trucks with humanitarian provisions has crossed into Ukrainian territory without Kiev’s permission.
Russia has previously been accused by Kiev and Western forces of supplying pro-Russian insurgents in Ukraine. However, this latest move signifies a more direct military involvement for Russia in Ukraine, if NATO's report is true. For its part, Russia claims, via Russian U.N. Ambassador Vitaly Churkin, that no Russian military forces are there.
Additionally, a 90-truck Russian convoy has crossed into Ukraine without Ukrainian permission or International Red Cross inspection. Russia insists that the trucks contain humanitarian aid, and the Russian Foreign Ministry issued a statement explaining, "The endless delays hampering the initial deliveries of the Russian humanitarian relief aid to southeastern Ukraine have become intolerable."
In a statement, the Pentagon said,
We strongly condemn this action and any of the actions that Russian forces take that increase tensions in the region. Russia should not send vehicles, persons or cargo of any kind into Ukraine, whether under the guise of humanitarian aid and convoys or any other pretext without Kiev’s express permission. This is a violation of Ukraine’s sovereignty and territorial integrity by Russia. Russia must remove its vehicles and its personnel from the territory of Ukraine immediately. Failure to do so will result in additional costs and isolation.
There was no specification as to what those additional costs or isolation would be.