The fatalities are among at least 10 civilians killed total, in addition to 20 injured, in the shelling of four city districts in the past 24 hours, the Donetsk City Council said on its website. Ukraine's state-run Ukrinform news, citing the Donetsk government, reported eight dead and seven hospitalized in the bus stop attack alone.
Donetsk has been a stronghold for separatists, who have been battling government forces for control of the Donetsk and Luhansk regions since the spring.
"As of now, the situation remains difficult," the city council said.
And as has been the case with many incidents in the conflict, there was no universally accepted explanation of what happened or who was responsible.
Russia's Foreign Ministry demanded an investigation and criticized Ukraine for its shelling of Ukrainian cities like Donetsk, which are controlled or at least full of pro-Russian separatists.
Yet Ukraine's Defense Ministry blamed the rebels for the attack, saying it was launched from areas they control.
The Organization for Security and Cooperation in Europe, which has a monitoring mission in Ukraine, is among those trying to get to the bottom of it all. Its deputy chief monitor, Alexander Hug, told reporters that an OCSE patrol saw seven bodies near a vehicle that had been hit.
And it's not the only instance of violence in the area.
"Terrorists using multiple launch missile systems precisely destroy(ed) houses" in the Donetsk region cities of Debaltseve and Avdiyivka, Ukrinform reported, citing Donetsk regional police Chief Vyacheslav Abroskin.
Rebels captured 16 wounded Ukrainian soldiers after fighting at Donetsk's airport, Kiev's military said, according to Ukrinform. Negotiations are underway to secure their release.
And the OSCE, in an update posted Wednesday
, cited numerous instances of shelling in recent days, with two hospitals in separatist-held territory among the buildings affected.
As U.S. Gen. Philip Breedlove, NATO's Supreme Allied Commander Europe, pointed out Thursday: "Violence has intensified and changed character in Ukraine."
Bloodshed follows peace talks in Berlin
As has been true so often in recent months, the violence is happening around the same time as negotiations aimed at stopping it.
The latest talks happened Wednesday in Berlin and included the foreign ministers of Ukraine and Russia, as well as Germany and France. Their official aim was to resolve the crisis by hammering out how to implement a peace deal that was agreed upon last year.
After Wednesday's talks, the four foreign ministers condemned the continuing violence and called for the peace deal that was reached in Minsk, Belarus, last September to be respected.
Fighting in the Donetsk and Luhansk regions "has severely escalated causing the loss of many human lives including civilians. This must stop immediately and the regime of quiet must be restored," the ministers said.
They said "tangible progress" must be made on full implementation of the Minsk pact ahead of a planned peace summit.
The attack on the Donetsk trolley bus station obviously doesn't help the situation. Russia's foreign ministry called it a "a crime against humanity (and) a great provocation aimed to undermine the efforts for the peaceful settlement of the crisis."
Poroshenko: '9,000 Russian troops in Ukraine'
The unrest began in late 2013, leading up to Parliament's ouster last February of pro-Moscow President Viktor Yanukovych. Russia annexed the Crimean Peninsula a few weeks later, with fighting breaking out in April in two Ukrainian regions bordering Russia -- Donetsk and Luhansk.
Separatist leaders there declared independence from Ukraine and sided with Russia. But how much was, and is, Moscow involved in their campaign?
It depends who you ask. Officials in Moscow have expressed sympathy for the separatists, but denied any outright military support. However, Ukraine and its allies have accused Russia of fomenting the rebellion by actively backing it.
"It is clear that Russia has to stop supporting the separatists and respect international law," said Danish Gen. Knud Bartels -- chairman of the military committee for NATO, which doesn't include Ukraine but is working to further its "military-to-military cooperation" with Kiev.
On Wednesday, Ukrainian President Petro Poroshenko reasserted a claim that Russia sent troops across the border to support the rebels
"We have more than 9,000 Russian troops who -- so-called -- lost the way, crossing our Russian-Ukrainian border, bring with them hundreds and hundreds of tanks, armed personnel carriers, and killing Ukrainian civilians and attacking Ukrainian troops," Poroshenko told CNN.
Russian Foreign Minister Sergey Lavrov, speaking in Moscow before the Berlin talks, responded to Ukraine's assertion with a firm denial -- much like other denials made by Russian officials in recent months over similar allegations of armed intervention.
"As to the flow of troops and armaments, this is not the first time we hear something like that," he said. "And each time I hear that I say, if you're so confident about that, please present us with facts, but no one has been able to provide us with these facts."