GENEVA, May 22 (Reuters) - The head of the World Health Organization urged countries to undertake the reforms needed to prepare for the next pandemic at the U.N. health agency's annual assembly on Monday.

"We cannot kick this can down the road," WHO Director-General Tedros Adhanom Ghebreyesus said in a major address to the agency's member states, warning that the next pandemic was bound to "come knocking".

"If we do not make the changes that must be made, then who will? And if we do not make them now then when?" he said.

The 10-day annual World Health Assembly in Geneva, which coincides with the body's 75th anniversary, is set to address numerous global health challenges including a key meeting this week on pandemics.