Three bridges into rebel-held Donetsk were destroyed Monday, as Ukrainian forces continue to fight pro-Russian separatists in the eastern part of the country
The bridges are access points from Slovyansk, which Ukrainian troops reclaimed from rebels Saturday.
A Reuters video shows a freight train stretched over what used to be a railway, now collapsed into a V-shape onto the highway below.
It is unclear which of the three bridges reportedly damaged by blasts on Monday is shown in the video.
A railroad worker told the news agency that villagers near one bridge heard an explosion.
There is no word yet on who is behind Monday's blasts.
The bridges were damaged near the towns of Zakotne, Novobakhmutivka, and Seleznivka along the roughly 100 kilometers from Slovyansk to Donetsk.
Separatists from Slovyansk are said to have retreated toward Donetsk, the east's main industrial hub where pro-Russian rebels first declared a "people's republic."
On Sunday, Ukrainian President Petro Poroshenko ordered military forces to "tighten the ring around the terrorists."
Pavlo Shisholin, deputy head of Ukraine's border control service, said in a news conference Monday that Russian forces continue their build-up along the border.
"Military equipment is continuing to build up on the territory of the Russian Federation. This includes tanks, APCs [armored personnel carriers], armored vehicles. For example, near the village of Valyiki, in the Kharkiv region, 10 helicopters arrived and are currently stationed. In the region of Milerovo, which is close to our border, a new tactical group is located. And such facts take place on a daily basis."
The government offensive in Slovyansk came after Poroshenko refused to renew a unilateral cease-fire and ordered the resumption of a government offensive on June 30.
Ukrainian forces said they now had full control of Slovyansk and the nearby town of Kramatorsk.
Local news agency Novosti Donbass said armed rebels were patrolling one of the main streets of Donetsk on Sunday.
Slovyansk has been the most important stronghold of the militants fighting government forces in mainly Russian-speaking eastern Ukraine, a focal point of tensions between the West and Russia.
The uprisings in eastern Ukraine erupted in April as rebels took over state buildings, built a powerful arsenal of seized weapons and declared their independence from Kyiv, calling the pro-European government illegitimate.
The crisis began when street protests ousted Moscow-backed Viktor Yanukovych in late February for rejecting a landmark political and trade deal with the European Union in favor of closer ties with Russia.
Russia subsequently annexed Crimea and separatist revolts against the new Kyiv authorities broke out with rebels, widely believed to be supported by Moscow, declaring "people's republics" and saying they wanted to join Russia.