Russian forces based on the border with Ukraine have been ordered to carry out fresh military exercises.
Defence minister Sergei Shoigu said the order was in response to raids by Ukrainian forces on pro-Russian checkpoints in the east of the country.
Raids by Ukrainian commandos around the town of Sloviansk on Thursday morning left at least two separatists dead.
Russia said the United States should make Kiev stop the attacks in the east, saying "nothing has been done to resolve this urgent crisis so far".
Pro-Russian separatists are occupying government buildings in a dozen eastern Ukrainian towns, defying the central government.
In Geneva on 17 April, Russia, Ukraine, the EU and the US agreed that illegal armed groups, including those who have seized public buildings, must return home.
NATO officials estimated the Kremlin has up to 40,000 troops along Ukraine's border.
