The UN health body's Global Advisory Committee on Vaccine Safety (GACVS) said that cases of myocarditis -- inflammation of the heart muscle -- and pericarditis -- inflammation of the lining around the heart -- had been reported in multiple countries, especially the US.

"The reported cases have typically occurred within days of vaccination, more commonly among younger males and more often following the second dose the of Covid-19 mRNA vaccines," the committee said in a statement.

After reviewing available data, the GACVS judged that "current evidence suggests a likely causal association between myocarditis and the mRNA vaccines".

Nevertheless, "the benefits of mRNA Covid-19 vaccines outweigh the risks in reducing hospitalizations and deaths due to Covid-19 infections", it added, noting that the inflammations are "very rare".

Most cases of myocarditis and pericarditis following vaccinations were "mild" and required only "conservative" treatment, the committee said, although it is observing the possible long-term effects.

US medical authorities warned last month there was a likely link between mRNA vaccines -- such as Pfizer and Moderna -- and myocarditis cases among younger recipients, while also saying the benefits continued to outweigh the risks.

The WHO said the European Medical Agency's pharmacovigilance committee, which tracks medicines' side effects, had also seen a "plausible causal relationship" in a review of the data this week.

Myocarditis is a rare disease which experts believe is usually triggered by a virus.

Most sufferers experience chest pain, and it is often treated with anti-inflammatory drugs and -- if needed -- additional oxygen.

Israel was the first country to report myocarditis among vaccine recipients in its fast-paced rollout of the mRNA shots.