The company has infuriated the bloc by saying it can deliver only a fraction of the doses it promised for the first quarter of the year.
It blames production issues at European plants, but the EU says doses made elsewhere should make up the shortfall.
The EU has been criticised for the slow rollout of its vaccinations.
The contract between the EU and AstraZeneca contains a confidentiality clause but the EU has asked the company to release the details nevertheless.
Reports said last week that the EU would get 60% fewer doses of the vaccine than promised for January-March 2021.
The EU reiterated its position after the two sides met on Wednesday evening to try to resolve the issue.
Health Commissioner Stella Kyriakides tweeted that the EU regretted the “continued lack of clarity on the delivery schedule”.
“We will work with the company to find solutions and deliver vaccines rapidly for EU citizens,” she said.
An AstraZeneca spokesman said the company had “committed to even closer co-ordination to jointly chart a path for the delivery of our vaccine over the coming months”.
Pfizer/BioNTech, which has an even bigger vaccine-production deal with the EU, is also experiencing delays.
French drug maker Sanofi has announced that it will help produce 125 million doses of the Pfizer/BioNTech jab by the end of the year.
The company will allow Germany-based BioNTech to use its facilities in Frankfurt from July, Sanofi said in a statement, having delayed the development of its own vaccine.
At a news conference earlier on Wednesday, Ms Kyriakides said UK factories, which have not experienced problems, were part of its deal with the company and had to deliver.
“The 27 European Union member states are united that AstraZeneca needs to deliver on its commitments in our agreements,” she said.
In an interview on Tuesday with Italian newspaper La Repubblica, AstraZeneca CEO Pascal Soriot said the contract compelled it to make its “best effort”, rather than obliging it to meet a set deadline for delivery of the vaccines.
Ms Kyriakides said this characterisation of the deal was “not correct or acceptable”, and called on the company to be “open and transparent” about its production of vaccines.
“We signed an advanced purchase agreement for a product which at the time did not exist and which still today is not yet authorised and we signed it precisely to ensure that the company builds a manufacturing capacity to produce the vaccine early so that they can deliver a certain volume of doses the day that it is authorised,” she said.
She added that the EU rejected “the logic of first-come first-serve”.
“That may work at the neighbourhood butcher’s but not in contracts, and not in our advanced purchase agreements.”
She appeared to be responding to Mr Soriot, who said that the UK had signed its contract with AstraZeneca three months before the EU and that this extra time had been used to “fix all the glitches we experienced” regarding the UK.
Meanwhile, UK Prime Minister Boris Johnson said: “I’m confident of our supplies and we’ll keep rolling out vaccines as fast we possibly can.
“I am very pleased at the moment that we have the fastest rollout of vaccines in Europe by some way.” (Courtesy BBC)