Europe's drugs regulator said on Friday it was looking into Johnson & Johnson's shot over reports of rare blood clots.

Four serious cases of rare blood clots with low platelets, one of which was fatal, have been reported after inoculation with J&J's vaccine from its Janssen unit, the European Medicines Agency (EMA) said.

It was the first news of EMA's probe of the J&J vaccine, while AstraZeneca's coronavirus vaccine has been dogged for weeks over possible links to rare blood clots in the brain and abdomen, which the EMA has confirmed. 

U.S.-based J&J said that it was aware of the reports of blood clots possibly related to its COVID-19 vaccine and others, and is working with regulators to assess the data and provide relevant information. 

"At present, no clear causal relationship has been established between these rare events and the Janssen COVID-19 vaccine," the company said in an emailed statement. 

The EMA also said that it was not yet clear if there was a link between the two vaccines and the reported conditions, and that its safety committee will decide if regulatory action such as label changes were needed. 

Some countries in Europe and Asia have restricted the use of AstraZeneca's vaccine, Vaxzevria, in younger people following an update by EU and UK regulators this week that found a link between the events and the shot. Regulators have said the benefits of the vaccine outweigh the risks. 

J&J's single-dose vaccine has been approved for EU use, but a rollout is yet to begin. It is mainly being used in the United States currently. 

The EMA also said five cases of capillary leak syndrome in people who received AstraZeneca's vaccine were reported. 

The condition, in which blood leaks from the smallest of vessels into muscles and body cavities, is characterized by swelling and a drop in blood pressure. AstraZeneca did not immediately respond to a request for comment. 

The J&J and AstraZeneca shots are two of the four approved COVID-19 vaccinations in Europe.

(With input from Reuters)