The situation in the anti-terrorist operation (ATO) zone in eastern Ukraine following Easter holidays deteriorated, OSCE Secretary General Lamberto Zannier has said.

"The ceasefire over Easter demonstrated that if there is a willingness, this can be done. Over Easter the sides agreed to make an effort and [the ceasefire] largely held. There were a few violations. So, if there is the will, it can happen. Our job is to pressure everyone to continue on that more positive behavior," he said in an interview in Kyiv on Monday.

He said that he was told that there were up to 1,000 violations of the ceasefire in some areas.

"So, in some areas we are really concerned. That is also in a way bringing a risk of a further escalation of the situation. We have been told that the lines are coming closer together, in some areas as close as between 100-150 meters, so this is, of course, inviting more hostilities," he stressed.

Illegal armed formations open fire using heavy weapons hidden in residential areas, he said.

"What we have seen, especially from the separatist side, is heavy weapons being used from residential areas. So this attracts military responses, which may hit residential areas and create civilian casualties," Zannier added.

The other negative aspect in the conflict he called hampering of free movements of the observers.

The OSCE secretary general said that his purpose of arrival to Ukraine is to discuss with OSCE monitors the current situation. He said no meetings with the country's leaders were on his agenda.