Four vehicles belonging to the OSCE monitoring mission to Ukraine were set on fire outside their hotel overnight in the rebel-controlled Donetsk, the observers said.
Organization for Security and Cooperation in Europe observers said in a tweet Sunday it was an apparent arson attack.
An OSCE statement said monitors heard four loud bursts about 2:30 a.m. local time, which they said were likely the sound of the tires on the armored vehicles exploding. Firefighters had extinguished the flames by 3:20 a.m.
Three OSCE vehicles were completely destroyed, one heavily damaged and three others partially damaged by the heat from the fire, the OSCE statement said. No observers were injured in the arson.
Mission began in 2014
The OSCE has had a mission in eastern Ukraine since March 2014 to monitor the conflict between the Russia-backed separatists and Ukrainian government troops, which has now killed at least 6,400 people.
Ukraine's military spokesman Oleksandr Motuzyanyk linked the attack to the OSCE's recent "unfavorable" statements on the conflict in the region and about "the unlawful armed groups who today have occupied Donetsk," a report by the French news agency AFP said.
"It's obvious that this was done by their partners in crime," Motuzyanyk said.
The rebels however blamed the attack on "Ukrainian saboteurs," according to their official website.
A senior rebel figure, Denis Pushilin, said the separatist authorities had "stepped up security measures for the mission representatives," quoted by the separatists' official news agency.
The OSCE mission said in statements on Twitter that despite the apparent arson attack, it had no plans to withdraw from Donetsk.
Some material for this report came from AP and AFP.