Ursula von der Leyen urged "transparency" from other countries, but did not confirm if the EU would bring in tougher export restrictions on coronavirus jabs.
This comes amid a row over supplies with the British-Swedish pharmaceutical giant.
However, von der Leyen acknowledged that worldwide supply chains needed to remain "intact" for vaccine production, while some European leaders appeared optimistic that the UK and EU could soon resolve their dispute over supplies.
Just days after the Prime Minister Boris Johnson flagged tougher measures to prevent Covid-19 variants being transmitted from Europe, the Telegraph reports the Government is planning to implement a mass testing regime for lorry drivers arriving in the UK.
Earlier, addressing a Brussels press conference following a meeting of the European Council on Thursday, Ms von der Leyen said she had "no knowledge" of the UK exporting jabs, while 77 million doses had been exported to 33 countries by the EU so far.
It came after the European Commission set out a tougher regime to stem supplies of jabs to nations faring better in the pandemic as the bloc's states faced a third wave of cases.
Ms von der Leyen told reporters: "Companies have to honour their contract to the European Union before they export to other regions in the world. This is of course the case with AstraZeneca.
"I think it is clear that the company (AstraZeneca) has to catch up and honour the contract it has with the EU member states before it can engage again in exporting vaccines."
She added: "We have worldwide supply chains that have to be intact and it is of the utmost importance that we get back to an attitude of openness."
Asked about how many vaccines the UK had exported, Ms von der Leyen told reporters: "I have no knowledge so far of UK exports, perhaps I am mistaken and waiting for their transparency."
Following the summit, Belgian prime minister Alexander de Croo said that he believed the EU's dispute with the UK over vaccine supplies "can be resolved" noting a phone call with Mr Johnson.
"We think that the discussion we have with the United Kingdom can be resolved based on good agreements," Mr de Croo told a Brussels press conference.
Dutch prime minister Mark Rutte also said that he was "cautiously optimistic" that divisions between the UK and the EU could be resolved.
"I think that on Saturday or soon after, they could come to an agreement which would be very helpful because we are friends, the UK and the rest of Europe, and we need each other," he told reporters.
However leaders appeared to be divided on the issue following the summit, with French president Emmanuel Macron insisting that the EU is "no longer naive" with its export controls tool.
Meanwhile, Mr Johnson acknowledged the "moral complexities" around a possible domestic coronavirus certificate and suggested that it might only be possible to introduce one after all adults had been offered a vaccine at the end of July.