The buses were hit by guided bombs, according to Ukrainian rescue services. About 20 people were injured.
At least four others have been killed elsewhere in the region. Donetsk Governor Vadym Filashkin accuses Russia of seeing buses as attractive targets because a single strike can cause many deaths. Russia "brings death to our country," he writes.
UN chief: Reprehensible
Ukrainian President Volodymyr Zelenskyy has shared a partially blurred video clip of the attack in Kramatorsk on social media. It appears to show burned bodies among debris on the road.
"This attack will be responded to. Russia is trying to kill all life in Donbas, and the world must respond. A strong response - exactly what the Russians deserve for all the death they have caused," Zelenskyy writes.
Kramatorsk is described as an important hub for supplying Ukraine's military, and the city is increasingly exposed to attacks as Russian forces creep closer.
Far from the front, in the capital Kyiv, at least six people were killed in Russian drone strikes, according to Mayor Vitali Klitschko. Many were injured.
“The manner and scale of these attacks - killing and injuring people on buses, in their homes and at work in different parts of the country - is reprehensible,” said UN human rights chief Volker Türk in a statement.
Survival mode in Kyiv
In Kramatorsk, regular traffic has been suspended, and in Kyiv, the metro is at a standstill because power has again been cut off as a result of Russian attacks on the Ukrainian power grid.
The capital has been forced into survival mode for the winter, Klitschko told the AFP news agency.
Every day there are new reports of buildings destroyed, of people injured or tragically killed, he says.
There has been no word from Moscow about Thursday morning's attacks, which followed a Wednesday in which about 30 people - including five children - were killed in major Russian strikes. In Pryluky, east of Kyiv, at least 22 people were killed when a Russian cruise missile demolished large parts of a five-story building.
Zelenskyy visited Pryluky on Thursday with visiting British Foreign Secretary Ed Miliband. In recent days alone, 78 people have been killed in Russian attacks on cities, Zelenskyy writes on X.
Attacks on civilian targets in Ukraine have increased dramatically. According to the UN, more Ukrainian civilians were killed in August than in all of last year, reports the BBC.
