KIEV (Reuters) - Four armored vehicles belonging to the OSCE monitoring mission in Ukraine's eastern conflict zone were destroyed overnight, the security watchdog said Sunday, describing the apparent arson attack as an attempt to restrict its work.

The Organization for Security and Cooperation in Europe monitors the implementation of a much-violated ceasefire deal and has faced increased hostility in separatist territory in recent weeks.

"It seems that this sort of intimidation is aimed at stopping the OSCE from reporting what is going on in the area," Alexander Hug, the deputy head of the monitoring mission, said in Kiev.

"There are, it seems, some who would like the OSCE to stop reporting what is going on in Donetsk," he said, adding that the organization had repeatedly asked those in effective power in the city to improve security for the mission.

The head of the self-proclaimed Donetsk People's Republic, Denis Pushilin, condemned the attack as an "outrage" and said all efforts would be made to find those responsible and ensure the safety of the mission.

"It's possible OSCE representatives have not always been objective due to various reasons ranging from the technical to the demands of high politics, however we absolutely are not interested in the mission ending its operation," he said in an online statement.

Hug said the mission had no plans to pull out of Donetsk.

The attack follows a protest of about 300 people outside the OSCE's hotel in rebel-held Donetsk on Thursday, when demonstrators held up banners criticizing the "silence and blindness" of the mission and sprayed red liquid from a hose onto the ground.

In July a similar protest was described by Hug as having been deliberately orchestrated.

The OSCE has regularly accused both separatists and Ukrainian troops of failing to respect the ceasefire and not fully withdrawing heavy artillery as required by the peace agreement, which was brokered in February to end a conflict in which more than 6,500 people have been killed.

On Sunday both sides accused the other 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, describing the situation in the area as unstable.

One Ukrainian soldier was killed, while 10 servicemen, a civilian woman and two children were wounded in the past 24 hours, Ukrainian military spokesman Oleksander Motuzyanyk said in a daily televised briefing.

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, north-east of Donetsk.