Bociurkiw said he had had only two such "incidents in front of him," and the recent one was during a visit to the northeastern part of Shyrokyne, which is the territory controlled by militants from the self-proclaimed Donetsk People's Republic.

"There are only two [such] incidents in front of me, that we have seen, of course, we will continue to gather as much information on this type of development as possible," he said at an OSCE SMM press briefing at Kyiv's Ukraine Crisis Media Center on Thursday.

"During the most recent visit [to Shyrokyne], our monitors did spot what appeared to be underage children at that checkpoint. This is the second time in recent times that we've seen children, and some of them [were] with arms," he said.

"We're very concerned about observing this type of development – any time when children are involved in a conflict is unacceptable," he added.

According to him, the monitors did not talk to the children.