UN agencies and aid organizations on Wednesday expressed concern about thousands of civilians, mainly elderly and vulnerable Ukrainian citizens, facing difficulties every day in crossing the so-called "contact" line, Farhan Haq, the deputy UN spokesman, told reporters here.

Neal Walker, the UN humanitarian coordinator in Ukraine, urged the government to keep checkpoints open, noting their closure has an immediate impact on people's lives, directly increasing hardship and humanitarian need, Haq said at a daily news briefing here.

Walker added that if hostilities increase, civilians may be trapped in unsafe areas, at the mercy of violence, mines and unexploded munitions.

For her part, Barbara Manzi, who heads the UN Office for the Coordination of Humanitarian Affairs (OCHA) in Ukraine, stressed that all parties to the conflict have an obligation under international humanitarian law to ensure the protection of civilians and to facilitate access to humanitarian organisations, Haq said.

Restrictions are also placed on people living in areas under government control close to the frontline. Closure, even if temporary, of one or more checkpoints will have severe consequences for these people and the government's decision to close Zaytseve checkpoint in the Donetsk region, starting Wednesday, and possibly other crossing points, is of serious concern.

International humanitarian law stipulates that if a certain transport corridor is closed, all alternative options need to be explored and new safe corridors established to ensure civilians can move freely, especially from areas of heightened hostilities.