France was treated in an "unacceptable" way when Australia, Britain and the United States negotiated a defence pact ditching its contract to supply submarines to Canberra, EU chief Ursula von der Leyen has said.

"There are a lot of open questions that have to be answered," Ms von der Leyen told CNN, according to a video extract of her interview.

"One of our member states has been treated in a way that is not acceptable."

Ms von der Leyen, President of the European Commission, said Brussels would want to "clarify" what had happened "before you keep on going with business as usual".

The full interview is due to be broadcast later today.

France reacted with fury last week when the three English-speaking allies revealed the AUSUK pact, which would see Washington supply Australia with nuclear-powered subs.

This torpedoed a long-standing agreement that France would supply diesel-powered boats, to be assembled in an Australian shipyard, in a multi-billion dollar deal.

Paris has recalled its ambassadors from Canberra and Washington for consultations, and EU foreign ministers have added the issue to their meeting later today in New York, ahead of the UN General Assembly.

In Brussels, diplomats said that the 27-member union was broadly behind France in its disagreement, but so far they have stopped short of publicly backing France's most aggressive comments.

Foreign Minister Jean-Yves Le Drian has said France was "stabbed in the back" by the pact, and European affairs minister Clement Beaune has suggested it could derail EU-Australian trade talks.