GENEVA (Reuters) - The head of a U.N. agency said on Monday that more than 500,000 people have fled Ukraine to neighbouring countries since the start of Russia's invasion last week.

       Filippo Grandi, head of the U.N. refugee agency UNHCR, made the remarks on his Twitter feed as the global agency's chief said its teams were stepping up humanitarian efforts amid escalating rights abuses there.

       (Reporting by Emma Farge, Editing by Louise Heavens)