The EU has urged pharmaceutical firm AstraZeneca to supply it with an increase of doses of its Covid-19 vaccine from UK plants, amid a row over shortages.
The business has infuriated the bloc by saying it could deliver only a fraction of the doses it promised for the first quarter of the entire year.
It blames production concerns at European plants, however the EU says doses made elsewhere should constitute the shortfall.
The EU has been criticized for the slow rollout of its vaccinations.
The contract between your EU and AstraZeneca contains a confidentiality clause but the EU has asked the business to release the details nevertheless.
Reports said the other day 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 attempt to resolve the issue.
Well being Commissioner Stella Kyriakides tweeted that the EU regretted the "continued insufficient clarity on the delivery schedule".
"We will function with the company to find alternatives and deliver vaccines rapidly for EU citizens," she said.
An AstraZeneca spokesman said the business had "committed to even better co-ordination to jointly chart a way for the delivery of our vaccine over the coming weeks".
Pfizer/BioNTech, which has a straight bigger vaccine-production handle the EU, is also experiencing delays.
French drug maker Sanofi has announced that it can 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 advancement of its own vaccine.
At a news conference earlier on Wednesday, Ms Kyriakides said UK factories, that have not experienced complications, were part of its deal with the firm and had to deliver.
"The 27 EU member states are united that AstraZeneca must deliver on its commitments inside our agreements," she said.
Within an interview on Tuesday with Italian newspaper La Repubblica, AstraZeneca CEO Pascal Soriot said the contract compelled it to create its "best effort", instead of obliging it to meet a set deadline for delivery of the vaccines.
Ms Kyriakides said this characterization of the offer was "not correct or acceptable", and called on the business to be "open and transparent" about its production of vaccines.
"We signed a sophisticated purchase agreement for something which at the time did not exist and which even so today is not yet certified and we signed it precisely to make certain that the company builds a manufacturing capability to produce the vaccine early in order that they can deliver a specific volume of doses your day that it's authorized," she said.
She added that the EU rejected "the logic of first-come first-serve".
"That may work at the neighborhood butcher's but not found in contracts, and not in our advanced purchase agreements."
She appeared to be giving an answer to Mr Soriot, who said that the UK had signed its agreement with AstraZeneca 90 days before the EU and that this extra time had been used to "fix all of the glitches we experienced" about the UK.
Meanwhile, UK Prime Minister Boris Johnson said: "I'm confident of our materials and we'll keep rolling away vaccines as fast we are able to.
"I am very pleased at the moment that we have the fastest rollout of vaccines in Europe by a way."