The ruble plunged more than 10 percent for the second day on Tuesday and recorded its worst fall since the Russian financial crisis in 1998 as confidence in the central bank evaporated after an ineffectual overnight rate hike.

Russia stepped up its criticism of the U.S. over Ukraine as President Barack Obama heads to eastern Europe to reassure NATO members of their security.

Determined to preserve the pro-Russian revolt in eastern Ukraine, Russia reinforced what Western and Ukrainian officials described as a stealth invasion on Wednesday, sending armored troops across the border as it expanded the conflict to a new section of Ukrainian territory.

A senior official in Kiev has accused Russia of invading Ukraine after Russian humanitarian aid lorries crossed the border without permission.

Ukraine accused pro-Russian separatists of killing dozens of civilians by firing on a refugee convoy, the latest example of residents caught in the line of fire of a four-month-old war that has caused more than 2,000 deaths.