"We are going into a second year of this, it could even be tougher given the transmission dynamics and some of the issues that we are seeing," Mike Ryan, the WHO's top emergencies official, said during an event on social media.
The worldwide death toll is approaching 2 million people since the pandemic began, with 91.5 million people infected.
The WHO, in its latest epidemiological update issued overnight, said after two weeks of fewer cases being reported, some five million new cases were reported last week, the likely result of a letdown of defenses during the holiday season in which people - and the virus - came together.
Maria Van Kerkhove, WHO's technical lead for COVID-19, warned: "After the holidays, in some countries the situation will get a lot worse before it gets better."
Amid growing fears of the more contagious coronavirus variant first detected in Britain but now entrenched worldwide, governments across Europe on Wednesday announced tighter, longer coronavirus restrictions.