Three bridges have been destroyed on roads leading into the city of Donetsk on Monday ahead of an expected offensive from government forces.
It was unclear who was behind the explosions but Ukrainian media said the bridges were blown up by rebels as Ukraine's military advanced.
Ukrainian troops regained control of the key rebel strongholds of Sloviansk and Kramatorsk on Saturday.
A journalist in Donetsk says civilians in the city are preparing for fighting.
"For the moment there are no gunfights but people are really scared," Yevgeny Shibalov told BBC Newshour.
"A lot of shops are closed and some banks officially declared they are closing their branches in the city," he added.
Separatist fighters driven out of Sloviansk and Kramatorsk at the weekend have regrouped in Donetsk.http://www.bbc.co.uk/news/world-europe-28191833