Turkey hit back Monday at claims of “blackmail” over a NATO plan for the Baltics and Poland, saying such allegations are out of the question.

“No one can question our contributions to NATO,” said Turkish security sources, underlining Turkey’s importance to the alliance on the eve of a NATO leaders’ summit in London due to start Tuesday.

Turkey, a member of the alliance for nearly 70 years, is at the very center of NATO with its contributions to the alliance, said the sources, who asked not to be named due to restrictions on speaking to the media.

Read more: ‘Claims of 'Turkish blackmail' unacceptable': Sources