Four armored vehicles belonging to the Special Monitoring Mission (SMM) of the Organization for Security and Cooperation in Europe (OSCE) were destroyed during the night in the eastern Ukrainian city of Donetsk.
The mission posted on Twitter on August 9 that the vehicles were "destroyed overnight in [an] apparent arson attack" at the mission's residence in Donetsk.
An SMM press release later in the day said the fire started around 2:30 a.m. local time. Three vehicles were totally destroyed, one was heavily damaged, and three others were partially damaged.
No one was injured.
Speaking to Reuters, deputy SMM head Alexander Hug called the incident an example of "intimidation aimed at stopping the OSCE from reporting what is going on in the area."
Russian media quoted Donetsk separatist leader Denis Pushilin as saying the incident was "an outrage" and pledging to do everything he could to find those responsible.
"We are absolutely not interested in the mission ending its operation [in Donetsk]," Pushilin said.
The SMM on August 7 reported a "significant increase" in cease-fire violations at and around the Donetsk airport.
The same day, about 300 people protested outside the SMM office in Donetsk, criticizing the "silence and blindness" of the monitors and painting a sidewalk and street with red paint.
The monitoring mission is charged with overseeing the implementation of the February Minsk agreements that established a cease-fire in the conflict between the Ukrainian government and Russia-backed separatists in the eastern part of the country.
Both sides have accused the other of repeatedly and regularly violating the cease-fire accords.
More than 6,500 people have been killed in the fighting in eastern Ukraine since April 2014.
Meanwhile, both sides accused the other on August 9 of shelling around the rebel-held town of Horlivka, north-east of Donetsk.
Separatists said one civilian had been killed by Ukrainian artillery fire on the town, while the Ukrainian military said rebels were shelling the positions of government troops nearby.
One Ukrainian soldier was killed, while 10 servicemen, a civilian woman and two children were wounded in the past 24 hours, a Ukrainian military spokesman said.
He said separatist civilian areas west of Donetsk city had been shelled constantly by rebels in the past day and reported an increase in fighting in parts of Luhansk region, northeast of Donetsk.
With reporting by Reuters, TASS, and Interfax