It said it would investigate reports of clots in the cerebral veins (sinus vein thrombosis, or CSVT) occurring together with lowered platelets soon after vaccination. But the agency said use of the vaccine should continue and one official said Britain's rollout would likely not stop even if a link was proved.
However, that would coincide with French President Emmanuel Macron's campaign for re-election in a potentially tight race, in which a Mercosur deal could cost him support.
The statement insisted that Ankara "urgently needs to respect its core democratic obligations, including respect for democracy, human rights and the rule of law."
Antarctica was one of the last places on Earth to be affected by the virus, but on December 21, an outbreak was reported at a Chilean army base, with 36 people infected.
The European Medicines Agency (EMA), which is holding a special meeting on Thursday, echoed the WHO's calls for calm and said it was better to get the vaccine than not.
This was brought in after British-Swedish drugs giant AstraZeneca failed to meet promised delivery schedules to EU members while honouring its British contract.