AstraZeneca will increase its coronavirus vaccine deliveries to the EU by 30 percent, European Commission chief Ursula von der Leyen said Sunday as the bloc sought to claw back time lost rolling out the jabs.

The British-Swedish company had announced last week that it could deliver only a quarter of the doses originally promised to the bloc for the first quarter of the year because of problems at one of its European factories.

But AstraZeneca, whose vaccine was authorised for use in the EU on Friday, has now agreed to send 9 million additional doses and "will start deliveries one week earlier than scheduled", Von der Leyen said in a tweet.

An EU source said the first deliveries would start in the second week of February.

AstraZeneca would also extend its production capacity in Europe, Von der Leyen added.

The EU leader was tweeting after talks Sunday with the leaders of the drugs companies that have signed vaccine contracts with the EU.

She told Germany's ZDF broadcaster the new doses represented an increase of 30 percent on the previous order.

"They are bringing forward the delivery now by another week ... and they will increase the vaccine doses for February and March by about 30 percent, that is 9 million doses," von der Leyen said.

But she also acknowledged that February and March would remain "a difficult phase" for vaccine supply. 

- EU vaccine supply pressures -

In the second quarter, the Johnson & Johnson vaccine will be on the market "and the manufacturers will have resolved their initial difficulties, so we can expect more vaccine", she said.

The aim was still to vaccinate 70 percent of adults in the EU by the end of summer, she added.

The EU has come under increasing pressure in recent days as it was forced to revise its original vaccination targets in the face of supply problems.

On January 19, it said it aimed to vaccinate 80 percent of health professionals and people aged over 80 by March.

But problems at AstraZeneca and with the Pfizer-BioNTech vaccine have threatened those plans.

Brussels has implicitly accused AstraZeneca of giving preferential treatment to Britain in the delivery of its vaccine, at the expense of the EU.

It last week demanded an inspection of the Belgian industrial site said to be responsible for the AstraZeneca delay, which is managed by a sub-contractor.

Germany's government on Sunday threatened any laboratory that failed to respect their obligations with legal action.

Top German officials are due to meet with the drugs manufacturers to thrash out the problems over the delays on Monday.