MOSCOW (AP) — Protesters in the rebel-controlled eastern Ukrainian city of Donetsk have vandalized cars from the Organization for Security and Cooperation and Security in Europe, the group said Friday.

Russian television on Thursday showed several dozen protesters outside the hotel where OSCE observers are staying to monitor a truce between the Russia-backed separatists and the Ukrainian government. Some of the protesters, mostly young men wearing sunglasses, spray-painted the cars of the OSCE and the International Committee of the Red Cross.

Alexander Hug, deputy head of the OSCE monitoring mission in Ukraine, said 30 cars owned by the OSCE and other international organizations have been damaged and are temporarily out of service.

Hug told reporters Friday in Donetsk that police were sent to the protest where the cars were vandalized “and did nothing to prevent this.”

“This act of violence is unacceptable,” said Hug, who has called for authorities to investigate.

Hug also said OSCE monitors continue to see heavy weapons from both sides near the front line, despite an agreement in February that both would pull the weapons back.

Rebels this month claimed they were beginning to pull back tanks and armored vehicles with smaller weapons as well; Hug said that while his observers have seen such vehicles moving, they cannot verify that an actual withdrawal is underway.

The fighting between separatists and Ukrainian forces has killed over 6,400 people since it began in April 2014.