Three bridges have been destroyed on roads leading into the city of Donetsk on Monday, July 7 ahead of an expected offensive from government forces, according to BBC news.

It was unclear who was behind the explosions but Ukrainian media said the bridges were blown up by pro-Russian activists as Ukraine's military advanced.

Ukrainian troops regained control of the key activist 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.

Pro-Russian activists driven out of Sloviansk and Kramatorsk at the weekend have regrouped in Donetsk.

A railway line in the village of Novobakhmutivka that crosses over a main road out of Donetsk was targeted, leaving a large cargo train dangling across the void.

A witness told the AP news agency that he saw a group of camouflaged men get into their cars and head towards Donetsk shortly before the bridge collapsed after an explosion.

Two other bridges on roads leading from Sloviansk to Donetsk were also destroyed in the villages of Zakitne and Seleznevka, Ukraine's transportation authority said on Monday.