A massive blast lights up a rebel stronghold in Ukraine (VIDEO)

KYIV, Ukraine — A massive explosion rocked the rebel stronghold of Donetsk late Sunday, the result of what some Ukrainian officials initially claimed was an artillery strike on a separatist weapons cache at a local chemical plant.

By Monday evening here, however, it was still unclear exactly what caused the explosion or whether there were any casualties.

A video clip purportedly of the blast has made rounds on social and mainstream media.

The clip appears to have been uploaded by a web user in the nearby city of Horlivka on Sunday. GlobalPost could not independently verify its contents.

Dmytro Yarosh, a parliament deputy and commander of a pro-Kyiv volunteer battalion, claimed on Facebook late Sunday that a Ukrainian military strike had wiped out 20 rocket launchers, 30 vehicles with weapons, and killed nearly 200 pro-Russian rebels. 

Another lawmaker, Borys Filatov, said on the same social network that security forces struck the area with a heavy rocket launch system.

So far, separatists have not yet claimed any fatalities, saying only that an unspecified number of people were injured in what they claimed was a Ukrainian strike. Rebel defense chief Eduard Basurin speculated it was the result of a tactical missile attack.

But on Monday, a Ukrainian General Staff spokesman told the Kyiv Post that officials were not even sure who attacked the state-owned factory, which is located in a western district of Donetsk. Meanwhile, another military spokesman claimed on Monday the blast could have come from a discarded cigarette butt.

The plant had reportedly come under attack at least once before, last fall.

While what happened Sunday still isn't clear, this is all adding to a tense atmosphere here. Battles have been escalating between Ukrainian forces and pro-Russian rebels, who launched an offensive last month to push the Ukrainian military farther back from their lines.

The United Nations says more than 5,000 people have been killed since the start of hostilities last April, with more than 1 million residents displaced.

German Chancellor Angela Merkel, who has played a key role in attempting to resolve the 9-month-old conflict, is in Washington today for talks with US President Barack Obama, as White House officials debate whether to provide Ukraine with lethal weapons.

More from GlobalPost: This infographic shows how deadly the war in eastern Ukraine is