GENEVA (Reuters) - Up to half a million Afghans could flee the crisis in their homeland, the U.N. refugee agency UNHCR said on Friday, appealing to all neighbouring countries to keep their borders open for those seeking safety.

       "In terms of numbers we are preparing for around 500,000 new refugees in the region. This is a worst case scenario," Kelly Clements, deputy U.N. High Commissioner for Refugees told a Geneva news briefing.

       "While we have not seen large outflows of Afghans at this point, the situation inside Afghanistan has evolved more rapidly than anyone expected," she added.

       (Reporting by Stephanie Nebehay and Emma Farge)