GENEVA, Dec 20 (Reuters) - A World Health Organization official said on Tuesday that 10 million people, or about a quarter of Ukraine's population, may suffer from a mental health disorder in relation to the conflict there."WHO estimates that up to 10 million people are at risk of some form of a mental disorder, varying from anxiety and stress to more severe conditions," Jarno Habicht, WHO's representative in Ukraine told a Geneva press briefing via video link.Ukraine's health care system has been under pressure since Russia invaded in February.So far, there have been at least 700 attacks on its health care system, WHO data shows, and Russia's increase in attacks on critical infrastructure since October has added to the challenges by causing blackouts.Habicht said respiratory diseases would increase with the cold weather and insufficient heating as would car accidents caused by unlit streets due to blackouts."