Ukraine's Defence Council said on Monday that preparations are under way to retake the rebel-controlled eastern city of Donetsk, which is the last stronghold of heavily armed pro-Russian separatists seeking autonomy from Kiev.
Defense spokesman Andriy Lysenko did not provide details, but he said the military has opened humanitarian corridors for city residents to flee the planned offensive, and that the government will help evacuees find temporary shelter.
As Lysenko spoke Monday, Moscow launched a massive show of military strength just inside Russian territory. Russian authorities say the five-day military exercise will involve 100 warplanes, helicopters and anti-aircraft batteries.
Russian officials said the military began exercises Monday in the central and western regions of the country, which would include the Ukraine border region.
The officials say the drills involving more than 100 fighter jets and helicopters will last through Friday.
While the announcement did not mention Ukraine, the military exercises could increase tensions in the already volatile region.
The United States and its European allies accuse Russia of arming pro-Russian separatists in eastern Ukraine, as well as building up military forces along the border with Ukraine and cross-border shelling.
On Sunday, deadly clashes between Ukrainian government forces and separatists continued as investigators searched the site of the crash of Malaysia Airlines Flight 17 for a third day.
Local officials in the city of Donetsk said six people were killed there in shelling between the two sides. Three other people were killed in Luhansk. The two eastern Ukrainian cities are the last remaining significant pro-Russian separatist strongholds.