Four armoured vehicles belonging to the OSCE in Ukraine have been destroyed in an apparent arson attack, the monitoring mission has said.
The overnight attack took place near the hotel where the observers are based in the rebel-held eastern city of Donetsk.
The observers are monitoring a fragile ceasefire between government forces and Russian-backed separatists.
But violence against them has increased in recent weeks.
The cars were set on fire early on Sunday morning, causing reinforced glass on their windows to crack and paint to peel off.
Ukrainian military spokesman Oleksandr Motuzyanyk blamed the attack on rebel “partners in crime”, while the separatists said on their official website that “Ukrainian saboteurs” were responsible for it.
The incident follows two protests at the mission’s headquarters in Donetsk. OSCE vehicles were damaged on one of these occasions.
OSCE officials said in a statement: “There are, it seems, some who would like the OSCE to stop reporting on what is going on in Donetsk.”
The OSCE has more than 500 observers throughout Ukraine, two-thirds of whom are based in the east.
The BBC’s David Stern in Kiev says they have come under increased pressure lately, and OSCE officials say they have been “targeted”.
But they say they have no intention so far of stopping their monitoring mission.