The WHO appealed to countries not to suspend vaccinations against a disease that has caused more than 2.7 million deaths worldwide. WHO Director-General Tedros said systems were in place to protect public health.
Portugal joined Spain, France, Germany, Italy, Denmark, Norway and several other countries in halting the use of the AstraZeneca jab amid concerns over possible serious side-effects.
The WHO said there was no evidence that the incidents are caused by the vaccine developed by AstraZeneca, an Anglo-Swedish company, with Oxford University.
The EU's EMA medicines regulator said it would meet on Thursday to analyse the situation and reaffirmed its view that the benefits of the drug outweigh the risks. The EMA will assess the information gathered into whether the AstraZeneca shot contributed to thromboembolic events (blood clots) in those inoculated.