According to reports, the leader of the Russian-backed separatists fighting in the eastern Donetsk region died in an explosion.
News agency Rebel DAN said the explosion that killed Alexander Zakharchenko, 42, passed through a café in the main city of the region.
Zakharchenko was prime minister of the self-proclaimed Donetsk People's Republic, which along with a separatist republic in neighboring Luhansk has fought against Ukrainian forces since 2014.
More than 10,000 people have died in the conflict.
The rebellion in Donetsk and Luhansk arose shortly after pro-Russian Ukrainian President Viktor Yanukovych was ousted from power amid massive protests in February 2014.
Russian speakers predominate in those regions, and separatist sentiment skyrocketed.
Encouraged by the annexation of Crimea in Russia, which also occurred after the overthrow of Yanukovych, the rebel leaders initially expected their regions to be absorbed by Russia as well.
Zakharchenko became prime minister of the DPR in August 2014.
Russian President Vladimir Putin praised Zakharchenko as "a true popular leader" and promised the residents of Donetsk that "Russia will always be with you."
The coffee in the city of Donetsk that was hit by the explosion, called Separ, had a separatist theme and had camouflage nets hanging from its eaves, showing recent photographs.
Denis Pushilin, the president of the separatist parliament, blamed Ukrainian forces for the explosion, calling it "the last aggression from the Ukrainian side," according to DAN.
A spokeswoman for the Ukrainian Security Service, Elena Gitlyanskaya, said the Ukrainian special services had no connection to the explosion.