The Hachette publishing group has reported a fall in ebook sales as it continues its battle with Amazon over pricing.

The publisher of Ian Rankin and Booker prize nominee Joshua Ferris has been at loggerheads with Amazon for several months, in a row that has led the online retailer to block pre-orders and delay deliveries of some of Hachette's bestselling authors in the US.

Lagardère, the French publishing company that owns Hachette, reported on Thursday that ebook sales in the US were down. Ebooks now make up 29% of adult book sales in this large market, down from 34% in June 2013.

The publisher said it had seen "a limited impact from Amazon's punitive measures", but added that the market for ebooks had reached a plateau.

Although Lagardère executives blamed a fall in the number of strong bestsellers for dragging down ebook sales, overall book sales in the US were up 5.6%, boosted by the sale of page-turners from Donna Tartt, former Navy seal Marcus Luttrell and Robert Galbraith, JK Rowling's crime-writing nom de plume.

In the UK, where Amazon has not deployed the same tactics against publishers, Hachette's ebook sales continued to grow.

Speaking on a conference call with market analysts, Dominique D'Hinnin, chief financial officer at Lagardère, played down the impact of the Amazon dispute. "Is there an impact in the Amazon's relationship? Maybe, but it is hard to tell."

In the latest twist in the dispute, Amazon said it was happy with its share of ebook revenues, but argued that Hachette should take less and give more to its writers – a move dismissed by some authors as a PR stunt.

The dispute burst into the open in May, near the end of Lagardère's reporting period, suggesting that a protracted row could weigh more heavily on ebook sales.

D'Hinnin said negotiations were "ongoing and active", but with no deadline to resolve the dispute. He was speaking as Lagardère reported a 1% dip in sales to €903m (£719m), reflecting a decline in the French market in a year without runaway successes, such as Fifty Shades of Grey or a Dan Brown novel. France is the latest market to suffer the Fifty Shades-effect, where book sales fade after a bumper year when novels from the erotic trilogy flew off the shelves.