Russian President Vladimir Putin has warned Ukraine is "on the verge of civil war."
The Kremlin said Putin made the comment in a telephone call with German Chancellor Angela Merkel on the situation there, RFE/RL reports.
Putin spoke hours after Ukrainian armed forces retook control of a small military air base in the east of the country, where pro-Russian separatists have seized control of buildings and other facilities in at least nine cities.
Acting President Oleksandr Turchynov said the base in Kramatorsk had been "liberated."
Another "anti-terrorist" operation was also reportedly under way in nearby Slovyansk, although few details have emerged there.
The White House described Ukraine's response to an "untenable" situation in the east as "measured."