Guardian Media Group is to cut costs in the US in response to challenging market conditions in an attempt to break even in the country by the next financial year.

Almost a third of jobs across all departments in the US are expected to be cut, reducing the number of staff employed in the country from 140 to about 100.

The group said market conditions made it “difficult but necessary” to make the changes if the Guardian was to break even in the US by 2017-18.

According to some estimates, technology groups such as Facebook and Google attract 85% of digital advertising spend in the US. Faced with such competition, Guardian revenues have failed to meet expectations despite having risen in the current financial year.

In an email to staff, the Guardian’s editor-in-chief, Katharine Viner, and the chief executive, David Pemsel, said the cuts were necessary even though strong audience growth in the US had led to the newspaper group being “among the top five digital newspaper platforms in America”.

The pair said all departments would feel the impact of the cuts but that the priority was “to protect Guardian journalism”.

Since a voluntary redundancy programme announced in March, the company has cut more than 260 jobs in the UK to save an estimated £17m a year.

In July, the Guardian reported losses of £69m for the last financial year but revealed strong growth in its membership scheme, with more than 50,000 people paying to sign up.

The Guardian US union committee said it was “deeply disappointed” with the decision, “considering the optimistic financial projections made to us within the last year”.

It acknowledged, however, that the management had said it would work with the union to decide how the cuts would be made.

“We are committed to fighting for fair and equitable treatment for the entire Guardian US editorial staff for as long as it takes,” the committee said.